Overlay marks, methods of overlay mark design and methods of overlay measurements

ABSTRACT

A method of designing an overlay mark, which is used to determine the relative position between two or more successive layers of a substrate or between two or more separately generated patterns on a single layer of a substrate, is disclosed. The method includes optimizing the geometry of a first element of the mark according to a first scale. The method further includes optimizing the geometry of a second element of the mark according to a second scale. The method additionally includes optimizing the geometry of a third element of the mark according to a third scale.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/894,987 filed on Jun. 27, 2001 which claims priority to U.S.Provisional Application No. 60/229,256 filed Aug. 30, 2001. Thisapplication also claims priority of U.S. Provisional Application No.60/301,484, filed Jun. 27, 2001. All of these applications are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to overlay measurementtechniques, which are used in semiconductor manufacturing processes.More specifically, the present invention relates to overlay marks formeasuring alignment error between different layers or different patternson the same layer of a semiconductor wafer stack.

The measurement of overlay error between successive patterned layers ona wafer is one of the most critical process control techniques used inthe manufacturing of integrated circuits and devices. Overlay accuracygenerally pertains to the determination of how accurately a firstpatterned layer aligns with respect to a second patterned layer disposedabove or below it and to the determination of how accurately a firstpattern aligns with respect to a second pattern disposed on the samelayer. Presently, overlay measurements are performed via test patternsthat are printed together with layers of the wafer. The images of thesetest patterns are captured via an imaging tool and an analysis algorithmis used to calculate the relative displacement of the patterns from thecaptured images.

The most commonly used overlay target pattern is the “Box-in-Box”target, which includes a pair of concentric squares (or boxes) that areformed on successive layers of the wafer. The overlay error is generallydetermined by comparing the position of one square relative to anothersquare.

To facilitate discussion, FIG. 1A is a top view of a typical“Box-in-Box” target 10. As shown, the target 10 includes an inner box 12disposed within an open-centered outer box 14. The inner box 12 isprinted on the top layer of the wafer while the outer box 14 is printedon the layer directly below the top layer of the wafer. As is generallywell known, the overlay error between the two boxes, along the x-axisfor example, is determined by calculating the locations of the edges oflines c1 and c2 of the outer box 14, and the edge locations of the linesc3 and c4 of the inner box 12, and then comparing the average separationbetween lines c1 and c3 with the average separation between lines c2 andc4. Half of the difference between the average separations c1&c3 andc2&c4 is the overlay error (along the x-axis). Thus, if the averagespacing between lines c1 and c3 is the same as the average spacingbetween lines c2 and c4, the corresponding overlay error tends to bezero. Although not described, the overlay error between the two boxesalong the y-axis may also be determined using the above technique.

There was also the introduction of the “Box in Bar” target and the “Barin Bar” target, both of which had the same general appearance as the“Box in Box” target. In “Box in Bar” targets, the outer box of the “Boxin Box” target is separated into a plurality of parallel bars (see FIG.1B). In “Bar in Bar” overlay marks, both the outer and inner box of the“Box in Box” target are separated into a plurality of parallel bars. Afurther example of this type of modified target is taught by Chen et. alin U.S. Pat. No. 6,118,185.

Recently, there was the introduction of separated bars that createdfeatures comparable to the design rules of the device itself. By way ofexample, Ausschnitt et al., in U.S. Pat. No. 6,130,750, discloses“Box-in Box” type targets having separated bars.

Although such designs have worked well, there are continuing efforts toprovide targets with improved functionality. For example, it would bedesirable to have targets capable of improving the correlation betweenthe overlay error measured on the test pattern and the real overlayerror of the circuit components.

SUMMARY OF THE INVENTION

The invention relates, in one embodiment, to a method of designing anoverlay mark which is used to determine the relative position betweentwo or more successive layers of a substrate or between two or moreseparately generated patterns on a single layer of a substrate. Themethod includes optimizing the geometry of a first element of the markaccording to a first scale. The method further includes optimizing thegeometry of a second element of the mark according to a second scale.The method additionally includes optimizing the geometry of a thirdelement of the mark according to a third scale.

The invention relates, in another embodiment, to an overlay mark fordetermining the relative shift between two or more successive layers ofa substrate. The overlay mark includes at least one test pattern fordetermining the relative shift between a first and a second layer of thesubstrate in a first direction. The test pattern includes a first set ofworking zones and a second set of working zones. The first set ofworking zones are disposed on a first layer of the substrate and have atleast two working zones diagonally opposed and spatially offset relativeto one another. The second set of working zones are disposed on a secondlayer of the substrate and have at least two working zones diagonallyopposed and spatially offset relative to one another. The first set ofworking zones are generally angled relative to the second set of workingzones thus forming an “X” shaped test pattern.

The invention relates, in another embodiment, to an overlay mark fordetermining the relative shift between two or more successive layers ofa substrate via an imaging device configured for capturing an image ofthe overlay mark. The overlay mark includes a first set of working zonesdisposed on a first layer of the substrate. The first set of workingzones includes at least two working zones diagonally opposed to oneanother and positioned within the perimeter of the mark. Each of theworking zones includes a periodic structure of coarsely segmentedelements positioned therein. The coarsely segmented elements aregenerally oriented in a first direction. The overlay mark furtherincludes a second set of working zones positioned crosswise relative tothe first working group. The second working group is disposed on asecond layer of the substrate and has at least two working zonesdiagonally opposed to one another and positioned within the perimeter ofthe mark. Each of the working zones has a periodic structure of coarselysegmented elements positioned therein. The coarsely segmented elementsare generally oriented in the first direction.

The invention relates, in another embodiment, to an overlay mark fordetermining the relative shift between two or more separately generatedpatterns on a single layer of a substrate. The overlay target includes atest region positioned on a first layer of the substrate. The firstlayer is typically formed by a first pattern via a first process and asecond pattern via a second process. The overlay target further includesa plurality of working zones positioned in the test region. The workingzones representing the actual areas of the test region that are used todetermine the relative shift between the first and second patterns. Afirst portion of the working zones are formed via the first process anda second portion of the working zones are formed via the second process.The overlay target additionally includes a periodic structure positionedwithin each of the working zones. Each of the periodic structuresincludes a plurality of coarsely segmented elements. Each of thecoarsely segmented elements are formed by a plurality of finelysegmented elements.

The invention relates, in another embodiment, to a method fordetermining the relative shift between two or more successive layers ofa substrate or between two or more separately generated patterns on asingle layer of a substrate. The method includes capturing an image ofan overlay mark formed on the substrate. The overlay mark having aplurality of working zones. Each of the working zones including aperiodic structure of coarsely segmented elements. The method furtherincludes selecting a plurality of working zones from the captured image,wherein at least one working zone from each layer or pattern isselected. The method additionally includes forming representativesignals for each of the selected working zones, wherein at least onesignal for each layer or pattern is formed. The method also includescomparing the signal from the first layer or pattern to the signal froma second layer or pattern to determine the relative shift betweendifferent layers or patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation.

FIGS. 1A&B are top plan views of overlay marks, which are well known inthe art.

FIG. 2 is a top plan view of an overlay mark, in accordance with oneembodiment of the present invention.

FIG. 3 is a partial side elevation view of a finely segmented periodicstructure, in accordance with one embodiment of the invention.

FIG. 4 is a partial side elevation view of a finely segmented periodicstructure, in accordance with one embodiment of the invention.

FIG. 5 is a partial side elevation view of a finely segmented periodicstructure, in accordance with one embodiment of the invention.

FIG. 6 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 7 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 8 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 9 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 10 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 11 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 12 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 13 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 14 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 15 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 16 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 17 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 18 is a top plan view of an overlay mark, in accordance with analternate embodiment of the present invention.

FIG. 19 is a simplified diagram of an overlay measurement system, inaccordance with one embodiment of the present invention.

FIG. 20A is a simplified flow diagram illustrating a method ofcalculating overlay, in accordance with one embodiment of the presentinvention.

FIG. 20B is a top plan view of the overlay mark of FIG. 9, in accordancewith one embodiment of the present invention.

FIG. 20C illustrates a pair of collapsed 1D signals, in accordance withone embodiment of the present invention.

FIG. 21 is a flow diagram illustrating a method 370 of calculatingoverlay using Covariance, in accordance with one embodiment of thepresent invention.

FIG. 22 is a flow diagram illustrating a method 380 of calculatingoverlay using Fourier Decomposition, in accordance with one embodimentof the present invention.

FIG. 23 is a flow diagram of a method of designing an overlay mark, inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps have notbeen described in detail in order not to unnecessarily obscure thepresent invention.

The present invention, in each of the various embodiments, uses overlaymarks that are composed of periodic structures formed on each of twolayers of a semiconductor wafer to provide overlay information betweenthose two layers of the semiconductor device. The overlay marks areformed in specific locations on each wafer layer such that the periodicstructures on one layer will be aligned with the periodic structures onthe other layer when the two layers are properly aligned. Conversely,the periodic structures on each layer will be offset from each otherwhen the two layers are not properly aligned. Alternatively, the presentinvention may use overlay marks that are composed of periodic structuresformed on a single layer by two or more separate processes to providealignment information between two different patterns on the same layer.Each of the periodic structures is composed of a plurality ofstructures, which increases the amount of information that may be usedto measure overlay, and which may be widely modified to diminish theimpact of certain processes on the overlay measurements. Each of thesestructures is composed of sub-structures that are about the same sizeand pitch (e.g., separation) as structures of the actual integratedcircuits. By forming each of the periodic structures with sub-structuresthat are sized closer to the size of the actual circuits, a moreaccurate measurement of any alignment error in such circuits isobtained. The invention is particularly suitable for overlay measurementtechniques that require capturing an image of the overlay mark.

The periodic structures and sub-structures described herein aregenerally patterned using suitable photolithographic techniques, and thelithographic patterns are subsequently transferred to other materialsand layers using established processing techniques such as etching anddeposition. In the simplest application, the transferred patternsconstitute etched or deposited lines or vias. For example, the periodicstructures and sub-structures may be formations of photoresist material,recessed cavity formations, embedded trenches and/or other structureswithin a wafer layer. The structures and sub-structures formed bycavities may be cavities formed in any of the layers during thesemiconductor fabrication process. For example, the cavities may beformed in the photoresist layer, the dielectric material layer, or themetal layers. It should be noted that the above processes are not alimitation and that any suitable fabrication technique may be used.

Embodiments of the invention are discussed below with reference to FIGS.1–23. However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these figures is forexplanatory purposes as the invention extends beyond these limitedembodiments.

FIG. 2 is a top plan view of an overlay mark 70, in accordance with oneembodiment of the present invention. The mark 70 is suitable for imagebased overlay measurement techniques. For ease of discussion, overlaymark 70 is shown in a configuration that results when the tested layersof a wafer are in proper alignment. The overlay mark 70 is generallyprovided to determine the relative shift between two or more successivelayers of a substrate or between two or more separately generatedpatterns on a single layer of a substrate. By way of example, theoverlay mark may be used to determine how accurately a first layeraligns with respect to a second layer disposed above or below it or howaccurately a first pattern aligns relative to a preceding or succeedingsecond pattern disposed on the same layer. For ease of discussion, theoverlay mark shown in FIG. 2 will be described in context of measuringoverlay between different layers of a wafer. It should be noted,however, that the overlay marks in this figure (and any subsequentfigures) may also be used to measure two or more separately generatedpatterns on a single layer of a wafer. In general, any convenientorientation of the mark relative to the placement of the dies on thewafer can be chosen so long as the orientation of the successive marksis the same from layer to layer or from pattern to pattern.

The overlay mark 70 is defined by a perimeter 71. The perimeter 71 istypically based on metrology tool limitations and circuit design rules.For instance, the upper limits of the perimeter may be set by the fieldof view (FOV) of the metrology tool used to measure overlay and/or thescribe line budget. The FOV generally refers to the optical perimeterthat defines the area available for capturing an image via the metrologytool. The mark is typically positioned inside the scribe line of thewafer, i.e., the scribe line is the place on the wafer where the waferis separated into dies via sawing or dicing and thus the circuit itselfis not patterned there. The scribe line budget, therefore, generallyrefers to the available space allowed by the scribe line for theplacement of the mark. In addition, the lower limits of the perimetermay be set by the minimum area needed by the metrology tool to image themark (e.g., obtain adequate signal or measurement quality).

It is generally believed that the perimeter 71 should be as a large aspossible so as to maximize the amount of information used for overlaymeasurements. The size and shape of the perimeter 71 may be widelyvaried. For example, the perimeter may form shapes such as squarescircles, triangles, rectangles, polygons and the like. The size of theperimeter, in any given direction, is generally between about 10 andabout 100 microns, and more particularly between about 20 and about 50microns. In the illustrated embodiment, the perimeter 71 directlycorresponds to the size and shape of the FOV 79 of the metrology tool,i.e., the FOV defines the perimeter. In most cases, the FOV isrectangular shaped due to CCD proportions. It should be noted, however,that this is not a limitation and that the FOV may be substantiallylarger than the mark's perimeter 71. For example, the mark's perimetermay be limited by the area on the scribe line.

The overlay mark 70 further includes a plurality of working zones 72,which are configured to divide and substantially fill the perimeter 71of the mark 70 (from center to edge). For example, the working zones maybe configured to fill the perimeter of the mark such that the combinedarea of the working zones is substantially equal to the total area ofthe mark. The working zones 72 represent the actual areas of the markthat are used to calculate alignment between different layers of thewafer. As such, the working zones 72 typically include informationrelating to the two layers for which overlay measurements are made. Forexample, some of the working zones are positioned in one layer of thewafer (represented by solid lines) while some of the working zones arepositioned in a different layer of the wafer (represented by dashedlines).

In most cases, the working zones 72 are spatially separated from oneanother so that they do not overlap portions of an adjacent working zone(i.e., each of the working zones represents a different area of themark). This is typically done to ensure that each of the working zonesis properly imaged by the metrology tool. Although not always necessary,the working zones 72 may be spatially separated by an exclusion zone 80so that each of the working zones is distinct. Exclusion zones 80 areareas of the target image where either a physical target structure or acorresponding optical signal is distorted and therefore it is excludedfrom the overlay calculation. The size of the exclusion zones istypically balanced with the size of the working zones so as to provideas much information as possible for the measurement of overlay. That is,it is generally desired to have larger working zones and smallerexclusion zones. In some cases, it may be desirable to have a smallamount of overlap between adjacent working zones so as to allow SEMcross sectioning to verify the accuracy of the overlay measurements.

In one embodiment, the geometry, including size, shape and distributionof the working zones is configured to balance out or compensate fornon-uniform or asymmetrical characteristics that may occur across themark. Asymmetries may arise from the optical aberration and illuminationasymmetry in the metrology tool (tool induced shifts), as well asprocess induced structural features (wafer induced shifts).

Tool induced shift (TIS) generally refers to how much the apparentposition of the mark moves or shifts as a result of metrology toolproblems such as non-uniform illuminations and/or optical aberrations.Illumination generally pertains to how the light is distributed to thetarget. Aberrations generally pertain to how the light is focussed andcollected. Non-uniform illuminations may be caused by a defect ormisalignment in one of the optical components associated withtransferring the light from the light source to the wafer. Non-uniformoptical aberrations may be caused by a defect or misalignment in theobjective lens of the optical system. By way of example, aberrations mayinclude spherical aberrations, astigmatism aberrations and comaaberrations. Coma aberrations may have a greater impact on TIS as aresult of its asymmetrical nature across the FOV of the metrology tool,

Wafer induced shift (WIS) generally refers to how much the apparentposition of the mark moves as a result of process variations such asdistortions caused by chemical mechanical polishing (CMP) and sputterdeposition.

Non-uniformities, asymmetries and variations may also arise fromdifference between the height of the different line sets from eachlayer. The lower layer lines are sometimes visible only through theintermediate layer of the wafer since the intermediate layer materialcovers the lines on the lower layer. On the other hand, the upper linesare generally formed from the photoresist applied on the top of thelower layer.

Accordingly, by adjusting the size, shape and distribution of theoverlay information from the two layers or patterns within the field ofview of the overlay metrology tool, it is possible to diminish theimpact of lithography and/or process non-uniformities, asymmetries andvariations. In some cases, it may even be possible to enhance theoverlay measurement. Resist patterns are less inclined to processvariation and thus they may potentially be more useful for acquisitionareas of the mark.

In one implementation, balancing is accomplished by selectivelypositioning the working zones of the same layer at different positionsaround the perimeter of the mark. For example, the working zones may bepositioned at different positions within the FOV in order to get thebest possible balance between aberrations and illumination and processresults that vary across the FOV. In the illustrated embodiment (FIG.2), working zones of the same layer are positioned opposite one anotherso as to balance out asymmetries that may occur from the left to theright or from the top to the bottom or from the inner to the outerregions of the FOV (or vice versa). In one implementation, the optimaldistribution for balancing out asymmetries is determined by performingoptical simulations of tool induced shift (e.g., how much the apparentposition of the mark moves as a result of tool problems such asillumination and/or aberrations). In another implementation, the optimaldistribution for balancing out asymmetries is determined by performingexperiments (e.g., run a sample of wafers). In addition, within the FOVthere might be parts of the mark, which have unacceptable aberrationsand illuminations and/or process damage, and therefore, the workingzones may be positioned at specific positions inside the FOV to avoidthese areas.

As mentioned, each of the working zones is configured to represent oneof the two successive layers of the overlay mark. In one embodiment, theworking zones represent an equal number of first layers or patterns andsecond layers or patterns, i.e., for each working zone in the firstlayer there is a corresponding working zone in the second layer. This istypically done to balance variations, non-uniformities and/orasymmetries that may exist in the layers and/or the metrology tool. Assuch, the number of working zones is generally based on a factor of 2,as for example, 2, 4, 8, 16 and the like. It is generally believed thatby distributing the regions to more points within the field of view, themore likely they are to balance out the non-uniformities caused byilluminations, aberrations and the process. By way of example, the sizeof the working zones (e.g., square) is generally between about 2 toabout 24 microns, and more particularly between about 4 to about 15microns. In most cases, the size of the working zones is inverselyproportional to the number of working zones, i.e., as the numberincreases, the size decreases. By way of example, the size of theworking zones is generally between about 10 and about 24 microns forfour working zones, between about 5 and about 12 microns for eightworking zones, and between about 2.5 and about 6 microns for sixteenworking zones.

In addition, there may be cases that require an uneven number of firstand second layered working zones, i.e., 4 first layered working zonesand 2 second layered working zones. There may also be the case thatrequires an uneven number of total working zones, i.e., 2 first layeredworking zones and 1 second layered working zones. There may also be thecase that requires unequal working zone sizes. For example, a firstgroup of working zones may have a first size while a second group ofworking zones may have a second size where the second size is eithersmaller or larger than the first size.

Although the working zones are generally constrained by the FOV (e.g.,perimeter of the mark), the shape of the working zones may varyaccording to the specific needs of each mark. By way of example, thezones may have a square shape (as shown), an L shape, a rectangularshape, a triangular shape, a circular shape, a polygonal shape and thelike. In most cases, the shape and size of the working zones areidentical. This is typically done to balance variations,non-uniformities and/or asymmetries that may exist in the layers and/orthe metrology tool. However, it should be noted, that this is not alimitation and that some or all of the working zones may have differentshapes. For example, some of the working zones may have a rectangularshape while other working zones may have a square shape.

Furthermore, working zones representing different layers are typicallyjuxtaposed relative to one another. By way of example, the mark mayinclude at least two juxtaposed working zones: a right regionrepresenting a first layer and a left region representing a secondlayer. In addition, the mark may include a top working zone representinga first layer and a bottom working zone representing a second layer. Insome implementations, the juxtaposed regions are positioned equidistantfrom the center of the target (e.g., center of FOV). It should be noted,however, that juxtaposition is not a limitation and that the position ofthe working zones may vary according to the specific needs of each mark.For example, there may be cases that require working zones representingfirst layers to be juxtaposed relative to other working zonesrepresenting first layers (or vice versa).

Referring to FIG. 2, working zones 72A and 72D (represented by dashedlines) are formed in one layer of the wafer while working zones 72B and72C are formed in a different layer of the wafer (represented by solidlines). As shown, working zones 72A&D are angled relative to workingzones 72B&C. That is, working zones 72A&D lie crosswise relative workingzones 72B&C. Furthermore working zones 72A and 72D, which are disposedon the same first layer, are positioned opposite one another at a firstvertical angle while working zones 72B and 72C, which are disposed onthe same second layer, are positioned opposite one another at a secondvertical angle. That is, working zone 72A is diagonally opposed toworking zone 72D, and working zone 72B is diagonally opposed to workingzone 72C. Moreover, working zone 72A is spatially offset from workingzone 72D, and working zone 72B is spatially offset from working zone72D. For example, the center of working zone 72D is positioned below andto the right of the center of working zone 72A, and the center ofworking zone 72C is positioned below and to the left of the center ofworking zone 72B. As should be appreciated, these cross-positionedstructures form an “X” shaped pattern.

It should be noted that this particular “X” configuration is shown byway of example and not by way of limitation, i.e., the size, shape anddistribution of the working zones and their periodic structures may varyaccording to the specific needs of each mark. For example, the workingzones may be configured to fill a variety of different sized anddifferent shaped FOVs. It is generally desirable to fill the field ofview with as much information as possible for reasons of processrobustness and information optimization. The working zones may also beconfigured to take on other shapes such as rectangles, triangles,parallelograms, trapezoids, regular polygons, circles and the like.Furthermore, the opposing periodic structures may be disposed on otherlayers. For example, working zones 72B and 72C may be disposed on thefirst layer (dashed lines) and working zones 72A and 72D may be disposedon the second layer (solid lines). Further still, the working zones mayonly partially fill the field of view. Moreover, the exclusion zones maybe eliminated so that the working zones may be positioned next to oneanother along their edges (e.g., completely filling the regions and thusthe FOV) or partially over one another so as to allow for line ends tooverlap for cross section accuracy.

Each of the working zones 72 contains an individual periodic structure74, as for example, periodic structures 74A–D. As shown, each of theperiodic structures 74 substantially fills the perimeter of itscorresponding working zone 72. Moreover, each of the periodic structures74 includes a plurality of coarsely segmented lines 76 that increase theamount of information that may be used for overlay measurements. Inaddition, by constructing marks from periodic structures, it is possibleto implement a broader range of overlay measurement algorithms thatmaximize the benefits of higher information density in the mark. Each ofthe coarsely segmented lines 76 is formed by a number of sub-structuresor finely segmented elements 78.

Even though some of the finely segmented elements 78 may be representedby dashed lines, the finely segmented elements 78 within each periodicstructure are not necessarily discontinuous linear formations that aresegmented at regular intervals. The dashed lines may representcontinuous linear formations within each of the periodic structures.However, in alternative embodiments, it is possible that the finelysegmented elements 78 within each periodic structure may take on variousshapes and sizes, which include discontinuous linear formations that aresegmented at regular intervals. These will be described in greaterdetail below.

In one embodiment, the geometry, i.e., linewidth and spacing, of theperiodic structure is configured to find the proper balance between theimage resolution of the metrology tool and the robustness of theprocess. For instance, in most cases, it is desirable to have a largegeometry (e.g., large linewidths and spacings) so that the periodicstructure may be optically resolved by the tool, and a small geometry(e.g., small linewidths and spacings) so that the process effects on themark are minimized. With regards to image resolution, there is a minimumsize requirement that each metrology tool has for resolving the coarselysegmented lines. Furthermore, it is generally known that as the periodof the periodic structure gets smaller, the metrology tool resolutiondiminishes, i.e., there is a point where the resolution of the metrologytool ceases to work effectively. With regards to process robustness,each time a new process is introduced in semiconductor manufacture,there is some impact on the overlay mark. The ability to measure thetarget depends on it's visibility or contrast in the image tool. Someprocesses such as metallization tend to diminish contrast, henceimpacting precision. Other processes such as chemical mechanicalpolishing (CMP) tend to blur or distort the mark, hence impactingaccuracy. These processes may also make the structures asymmetric orcreate an apparent, optically measured spatial translation of the centerof the structures relative to the center of the originally patternedtrench or line (e.g., circuit pattern).

For specific processes, such as aluminum coated, chemically mechanicallypolished tungsten, it is advantageous for the characteristic dimensionsof these structures to be approximately 1 to 2 microns or less in orderto diminish the impact of asymmetries resultant from the polishing andaluminum deposition processes. However, if the width of the trench istoo small, the remaining topography at the top of the aluminum layer istoo small to provide optically adequate contrast and thus the mark doesnot provide adequate overlay information. On the other hand, the lowerbound for characteristic dimensions of structures at this scale isdetermined by the resolution limit of the metrology tool. For examplefor an overlay tool with a numerical aperture (NA) of 0.9 and a meanillumination wavelength of 550 nm gives a Raleigh resolution limit orcriteria of approximately 0.4 microns. In this particular case, it maybe preferable to maintain the linewidth above 0.5 microns in order notto diminish contrast and hence signal to noise, and below 1–2 micron inorder to diminish the impact of asymmetries resultant from the polishingand aluminum deposition processes. It should be noted, however, thatthis is by way of example and not by way of limitation and that it maybe possible to achieve better that the Rayleigh resolution limit.

In one embodiment, the geometry of the periodic structure is determinedby experimentation. For example, several wafers may be run through aprocess to find a period for which the TIS variability is the smallest,i.e., measure the tool induced shift variability at multiple sites onthe wafer and then select the pitch that minimizes the TIS variabilityand process variability.

In one embodiment, the period and phase of the periodic structure isconfigured to filter out high frequency edges.

In the illustrated embodiment, each of the periodic structures 74A–D hasthe same pitch and duty cycle. That is, each of the periodic structures74 consist of an equal number of coarsely segmented lines 76, which areparallel and which have equal linewidths and equal spacingstherebetween. The dimensions of the pitch, linewidths and spacings maybe widely varied. By way of example, the dimension of the pitch may bebetween about 1 to about 3 microns, and the dimensions of the linewidthsand spacings may be between about 0.3 to about 2 microns, and moreparticularly between about 0.5 to about 1 micron.

It should be noted that equal pitch, linewidths and spacings for each ofthe periodic structures is not a limitation and that they may varyaccording to the specific needs of each mark. For example, each of theperiodic structures may have a different pitch or duty cycle.Alternatively, some of periodic structures may have the same pitch orduty cycle, while other periodic structures may have a different pitchor duty cycle. Furthermore, the periodic structures may have a pitchthat varies across the periodic structure. By way of example, theperiodic structure may be a chirped periodic structure (e.g., smaller tolarger). As was explained previously, the pitch, linewidths and spacingsare generally optimized according to process robustness requirements andcontrast requirements of the metrology tool.

The number of lines inside each periodic structure may be varied to meetthe specific needs of each mark. It is generally believed that thenumber of lines is dependent on the resolution required and the signalto noise ratio desired. Most imaging tools have a resolution limitbetween about 0.3 and about 0.9. The number of lines may also bedetermined by process requirements such as chemical mechanical polishingdistortions that affect the outermost line segments more than the innerline segments of a group. One factor affecting the maximum number oflines that may be used within a group of line segments is the metrologytool resolution. From the perspective of the minimum number of linesthat is needed for operation, that number is two. In the embodimentshown, each of the periodic structures 74A–D includes 5 coarselysegmented lines. In some cases, it may even be desirable to haveperiodic structures having a different number of lines, i.e., a firstperiodic structure having 5 lines and a second periodic structure having2 lines.

In the embodiment shown, the lines of the periodic structures 74A–D areparallel to one another so as to provide position information in asingle direction. As should be appreciated, the lines in FIG. 2 areconfigured for X-axis measurements since the lines are non-parallel(e.g., perpendicular or orthogonal) to the axis of measurement. Giventhis configuration, any offset between the two successive layers in theX-direction will be present between the first set of periodic structures74 A&D and the second set of periodic structures 74 B&C. As such, thealignment between the two layers of the wafer in the X-direction may bedetermined by comparing the relative positions of the two groups ofperiodic structures. For instance, the positions of periodic structures74 A and D, which are disposed on the first layer, may be compared withthe positions (e.g., centers of symmetry) of periodic structures 74 Band C, which are disposed on the second layer, to determine thealignment between consecutive layers in the X direction.

In one embodiment, the overlay alignment between layers is determined bycalculating the centers of symmetry for each of the opposing zones onthe same layer, and then calculating the difference between the twoaveraged centers of symmetry. For example, the center of symmetry forboth working zones 72A and D and working zones 72B and C may be found byfolding the images over, and the difference of these two centers ofsymmetry may determine the overlay error. If there is zero overlay, thecenters of symmetry of each of the two opposing groups on the same layershould coincide with the Y axis that runs through the middle of themark, i.e., between the left working zones and the right working zones.

Alternatively, periodic structures that are lateral to one another andon different layers are compared, i.e., periodic structure 74A iscompared to periodic structure 74B and periodic structure 74C iscompared to periodic structure 74D. In addition, periodic structuresthat are above or below one another are compared, i.e., periodicstructure 74A is compared to periodic structure 74C and periodicstructure 74B is compared to periodic structure 74D.

It should be noted that measuring overlay in the X-direction is not alimitation and that the overlay mark 70 may be rotated 90 degrees todetermine the registration error between the two layers of the wafer inthe Y-direction. Furthermore, two overlay marks, one of which is rotated90 degrees from the other, can be used to determine the alignmentbetween consecutive layers in two directions, as for example in the Xand Y directions. The second mark may be positioned at various locationsrelative to the first mark so long as the orientation of the first andsecond marks are the same layer to layer (e.g., side by side or atdifferent locations on the wafer if space is limited).

Note additionally, that if the lines shown in solid outline are printedon the first layer of the semiconductor wafer with the lines shown solidon the second layer, then on the third layer another set of lines (shownhere in solid outline) are printed over, and covering, the lines of thefirst layer. Then the lines of the second layer are used in conjunctionwith lines on the third layer. Thus, each set of lines on a layer of thesemiconductor wafer (except for those on the first and last layers) areused in conjunction with the lines on two layers of the semiconductorwafer, the one below and the one above. This implementation works bestif the first layer cannot be detected optically when below the thirdlayer. Alternatively, if there is sufficient space on the semiconductorwafer surface, the grating pairs for each pair of adjacent layers on thewafer could be in a different location on the wafer to minimize any“bleed through” interference from a third layer on the measurement forthe top two layers of interest.

To elaborate further, finely segmented elements 78, which are used toform each of the coarsely segmented lines 76, are configured to allowthe overlay mark 70 to facilitate overlay measurements that moreaccurately represent the degree of alignment between the wafer layers.That is, the finely segmented elements 78 serve to provide alignmentinformation that more closely matches the alignment of the patterns ofthe integrated circuits that are formed on each of the two layers. Thefinely segmented elements 78 allow for more representative measurements,in part, due to several reasons.

One reason for which smaller overlay marks provide more accurate overlaymeasurements is that the smaller sized finely segmented elements areformed on the semiconductor layer with lens pattern placement errorsthat are more similar to the lens pattern placement errors with whichthe patterns for the integrated circuits are formed. Patterns are formedon wafer layers with lithographic devices such as “steppers.” The lensplacement errors of patterns formed upon a semiconductor wafer changewith the size and spacing of the patterns due to aberrations within thestepper lenses, and with the illumination conditions (including off-axisillumination and partial coherence) used to expose the circuit patterndefined on the lithographic mask. Creating marks having feature size andpitch more comparable to that of the integrated circuit element criticaldimensions, as well as using the same or similar mask pattern techniquesas the circuit features (e.g., using the same or similar opticalproximity correction or phase shift mask patterns), results in mark andintegrated circuit patterns that are formed with a more similar degreeof lens pattern placement errors. In this manner, the alignment betweenmarks on different layers of a wafer gives a more accurate indication ofthe alignment between the circuit patterns. For more informationregarding distortions due to stepper lens aberrations, see LithographyProcess Control, by Harry J. Levinson.

In one embodiment, the feature size and pitch (e.g., the distancebetween the centers of the finely segmented elements of the finelysegmented elements are substantially equal to those of the criticaldevice features of the patterning step performed on the layers undertest. That is, the dimensions of the finely segmented elements 78 arecomparable to the dimensions of the circuit patterns. In oneimplementation, the line has a width that is approximately equal to thewidth of an integrated circuit interconnection line. Currently, circuitinterconnection lines have widths that are approximately equal to orless than 0.13 μm. The finely segmented elements of the currentinvention can be made to have widths as small as 0.05–0.2 μm. However,as can be appreciated, advances in semiconductor manufacturing processesare likely to further reduce these dimensions and therefore thesedimensions are by way of example and not by way of limitation.

Another reason for which smaller overlay marks provide more accurateoverlay measurements is that effects of wafer fabrication asymmetries onoverlay measurement may be reduced. Wafer fabrication asymmetries areshifts in the shape and size of structures or patterns that have beenformed upon a wafer layer due to further fabrication processes. Theeffects of wafer fabrication processes on overlay marks depends on thesize, spacing and density of the overlay mark structures andsubstructures. These shifts in shape and size affect the overlay markssuch that accuracy of the overlay measurements may be deteriorated.

An exemplary wafer fabrication technique that may cause wafer structuresto gain asymmetrical profiles is the sputter deposition process. Thesputtering process is generally used to apply a layer of material (i.e.,metal) on top of an existing wafer layer. Usually, the source of thesputtered material, a target, is located above the center of the wafer.The sputtered material travels at an angle from the target towards theouter perimeter of a wafer thereby resulting in asymmetrical depositionof material within recessed channels or over ridge-like protrusions.Specifically, the unequal accumulation of deposited material between thesidewalls of a recessed channel may cause an apparent positional shiftof the recessed channel towards one side of the channel.

Another exemplary fabrication technique that may cause asymmetricaldimensions is the chemical mechanical planarization (CMP) of waferlayers. In certain circumstances, wafer layers undergo CMP before thenext layer of material is deposited. The CMP device generally travelsover a wafer layer in a specific direction. The CMP device, therefore,will first encounter one side of an overlay mark and then run down theopposite side of the mark. This results in a shift and change in theapparent size of the overlay mark since the material on the side of theoverlay mark which is encountered first may be removed to a greater orlesser degree than the opposite side of the mark.

In both situations, the resulting asymmetries to overlay marks due tothe fabrication processes may be reduced by forming smaller marks. Withrespect to the sputtering process, smaller recessed channels or ridgesallows less sputtered material to accumulate on the respective sidesurfaces, thereby resulting in a smaller asymmetrical shift in shape andsize. With respect to the CMP process, marks having smaller dimensionswill also be shifted to a lesser degree. Conversely, there may beprocess situations, where widening the lines of the overlay marks maymake them more robust to process variation, as for example, in cases ofmetal layers with large grain size. Refer to Lithography ProcessControl, by Harry J. Levinson, for further information on waferfabrication asymmetries.

In the illustrated embodiment, the finely segmented elements 78 appearas thin lines that are spatially separated and parallel to each other.It should be noted, however, that lines are not a limitation and thatthe shape of the finely segmented elements may vary according to thespecific needs of each mark. For example, the finely segmented elementsmay be composed of squares, rectangles, triangles, polygons, circles,ovals and the like. As should be appreciated, the finely segmentedelements 78 may not have perfectly symmetrical shapes since they aretypically formed via lithographic and pattern transfer processes.

Further variations include variously shaped elements that are formedwithin a single overlay mark. For example, one periodic structure maycontain linearly shaped elements while a different periodic structuremay contain circularly shaped elements. In addition, one periodicstructure may contain circularly shaped elements and a differentperiodic structure may contain square shaped elements. Moreover, oneperiodic structure may contain linearly shaped elements and a differentperiodic structure may contain square shaped elements. Even furthervariations include variously shaped elements that are formed within asingle periodic structure. For example, a single periodic structure mayinclude one coarsely segmented line that is formed by linearly shapedelements and another coarsely segmented line that is formed by squareshaped elements. Even further variations, include some periodicstructures which are composed of finely segmented elements and otherswhich are not composed of finely segmented elements, but rather singlesolid lines.

Referring to FIG. 3, the finely segmented elements 78 will be describedin greater detail. FIG. 3 is a partial side elevation view of any one ofthe periodic structures 74 shown in FIG. 2, in accordance with oneembodiment of the invention. As shown, the coarsely segmented lines 76are formed by a plurality of finely segmented elements 78. In thisparticular embodiment, the finely segmented elements 78 represent bars,which are symmetrically distributed relative to the center of thecoarsely segmented line 76, and which having equal fine widths, w, andfine pitch, p (distance between centers), therebetween. It should benoted, however, that this is not a limitation and that the widths andpitch, as well as the distribution, may vary according to the specificneeds of each device. The linewidth, d, of the coarsely segmented lines76 is defined as the distance between the outer edges of the very firstand the very last bar 78′ and 78″ of the plurality of finely segmentedbars 78. In the illustrated embodiment, there are 10 finely segmentedbars.

It has been found that stepper aberrations (not metrology toolaberrations) such as coma may cause an apparent overlay error in caseswhere the overlay mark is finely segmented. That is, in addition tohaving the tendency to cause pattern placement errors, stepper coma mayalso have the tendency to modify the dimensions of the finely segmentedelements, in particular, the first and last bars that make up thecoarsely segmented line (e.g., bars 78′ and 78″). By “modify thedimensions”, it is generally meant that the first and last bars maybecome thinner or wider. In most cases, when one bar becomes wider theother bar becomes thinner. For example, coma may cause the last bar tobecome wider and the first bar to become thinner. This may also becaused by the proximity of the bars to the open space between coarselysegmented lines. As should be appreciated, this type of modificationtends to introduce an apparent overlay shift (i.e., the line appears toshift from the left to the right) and thus the line may not be measuredcorrectly. In one embodiment, the layout of the periodic structures maybe reconfigured to compensate for this apparent shift (see FIGS. 4 & 5).

In lithography, a clear field generally refers to a series of periodicstructures that are surrounded by an open space (e.g., etched) and adark field generally refers to a series of periodic structures that aresurrounded by a closed space (e.g., not etched). The clear fieldsgenerally appear brighter and the dark fields generally appear darker.In this particular embodiment, the clear field is a clearing between agroup of finely segmented elements such as the bars of FIG. 3, and thedark field is a closed space between a group of finely segmentedelements such as the bars of FIG. 3. It is generally believed that theclear fields and dark fields may be configured to alter the formation ofthe lines in such a way that that the apparent shift, which is caused bythe wider and thinner bars, cancel or balance out. In oneimplementation, the balancing is done within each periodic structure.For example, each of the working zones includes fine segmentation thatcomprises both a clear field and a dark field. For example, some of thecoarsely segmented lines have clear fields and some have dark fields. Inanother implementation, the balancing is done between two periodicstructures of different working zones. For example, at least a firstworking zone includes a periodic structure with fine segmentation thatcomprises a clear field and at least a second working zone includes aperiodic structure with fine segmentation that comprises a dark field.In most cases, the first and second working zones are opposed workingzones that are on the same layer. These implementations will bedescribed in greater detail below with reference to FIGS. 4 and 5.

FIG. 4 is a partial side elevation view of any one of the periodicstructures 74 shown in FIG. 2, in accordance with one embodiment of theinvention. By way of example, FIG. 4 generally corresponds to the firstimplementation described above. As shown, the coarsely segmented lines76 are formed by a plurality of finely segmented bars 78 and at leastone dark field 81. The coarsely segmented lines 76 are separated by aseparation, S, that includes a plurality of finely segmented bars 78 andat least one clear field 82. The geometry of the finely segmented lines,dark fields and clear fields may be widely varied. The geometry of thesecomponents generally depends on the partial coherence of illuminationand coma aberrations of the optics of the stepper lens. Dual tonestructures, in which both lines and spaces are partially segmented tendto represent better the pattern placement errors suffered by devicestructures, rather than structures comprising either one or the other.

FIGS. 5A&B are partial side elevation views of two distinct periodicstructures 84 and 85, in accordance with one embodiment of theinvention. In most cases, the periodic structures 84 and 85 representperiodic structures, which are from opposing working zones, and whichare positioned in the same layer. For example, periodic structure 84 maycorrespond to periodic structure 74A while periodic structure 85 maycorrespond to periodic structure 72D. It should be noted, however, thatthis is not a limitation.

Referring to FIG. 5A, the first periodic structure 84 includes coarselysegmented lines 76, which are formed by a plurality of finely segmentedbars 78, and which are separated by clear fields 82. Referring to FIG.5B, the second periodic structure 85 includes coarsely segmented lines76, which are formed by a plurality of finely segmented bars 78, andwhich are separated by dark fields 81. The geometry of the finelysegmented lines, dark fields and clear fields may be widely varied. Inthis particular embodiment, the size shape (although inverse) andposition of the dark fields generally corresponds to the size shape(although inverse) and position of the clear fields.

As can be seen from the foregoing, the advantages of the X-configurationare numerous. For instance, the X-configuration may provide moreinformation than the standard box-in-box target by filling the perimeterof the mark from center to edge. It may also provide more information byincreasing the number of edges (e.g., coarsely segmented lines) as wellas their lengths. Apart from providing more information by increasingthe number of edges and their lengths, the X-configuration exhibitsfurther advantage over the generic box-in-box target due to additionalbuilt-in symmetry. Namely, in the generic box-in-box structures thereare inner and outer layers. Their swap would result in differentpattern, and therefore—overlay result, due to different informationdistribution of inner and outer marks, and due to different opticalbehavior of the metrology tool in regions close to and far from the FOVcenter. From this point of view, the “X”-target structure is basicallyinvariant to the layer swap (up to mirror transformation). Furthermore,the X configuration exhibits 180 degree rotational symmetry that helpsto overcome anti-symmetrical coma patterns, i.e., coma will cancel out.

FIG. 6 is a top plan view of an overlay mark 90, in accordance with analternate embodiment of the invention. By way of example, the overlaymark 90 may generally correspond to the overlay mark shown in FIG. 2.Overlay mark 90 contains four working zones 92A–D for determining theregistration error between two wafer layers (one layer is represented bycross-hatching, the other is not). Each of the working zones includes aperiodic structure 94 comprised by a plurality of coarsely segmentedlines 96. In a manner similar to the X target of FIG. 2, the periodicstructures 94A and D positioned in the first and fourth zones 92A and Dare disposed in a first layer of the wafer while periodic structures 94Band C positioned in the second and third zones 92B and C are disposed ina second layer of the wafer. Furthermore, periodic structures on thesame layer, as for example structures 94 A and D, are in diagonallyopposed positions thereby forming an overlay mark with an Xconfiguration.

In this particular embodiment, the coarsely segmented lines arehorizontally positioned and therefore they are configured to measureoverlay in the Y-direction. It should be noted, however, that this isnot a limitation and that the target may be rotated so as to measureoverlay in the X-direction. Also in this embodiment, the coarselysegmented lines are solid structures that are elongated and rectangularin shape. It should also be noted that this is not a limitation and thatthe coarsely segmented lines may be formed by a plurality of finelysegmented elements, which may be produced according to the finelysegmented elements described in FIG. 4.

In contrast to the mark of FIG. 2, each of the working zones 92, andmore particularly each of the periodic structures 94 in FIG. 6 has an “Lshaped” outline or shape in order to accommodate an additional structure96 in the center of the mark 90. In the illustrated embodiment, theadditional structure 96 represents a standard box in box structure asdescribed in the background of this application. As such, the “X”configured mark 90 can be acquired and measured by standard box in boxmetrology tools and algorithms, i.e., existing equipment and softwaremay be used.

FIG. 7 is a top plan view of an overlay mark 100, in accordance with analternate embodiment of the invention. By way of example, the overlaymark 100 may generally correspond to the overlay mark shown in FIG. 2.Overlay mark 100 contains four working zones 102A–D for determining theregistration error between two wafer layers (one layer is represented bycross-hatching, the other is not). Each of the working zones 102includes a periodic structure 104 comprised by a plurality of coarselysegmented lines 106. In a manner similar to the “X” target of FIG. 2,the periodic structures 104A and D positioned in the first and fourthzones 102 A and D are disposed in a first layer of the wafer whileperiodic structures 104 B and C positioned in the second and third zones102 B and C are disposed in a second layer of the wafer. Furthermore,periodic structures on the same layer, as for example structures 104 Aand D, are in diagonally opposed positions thereby forming an overlaymark with an X configuration.

In this particular embodiment, the coarsely segmented lines 106 arehorizontally positioned and therefore they are configured to measureoverlay in the Y-direction. It should be noted, however, that this isnot a limitation and that the mark 100 may be rotated so as to measureoverlay in the X-direction. Also in this embodiment, the coarselysegmented lines are solid structures that are elongated and rectangularin shape. It should also be noted that this is not a limitation and thatthe coarsely segmented lines may be formed by a plurality of finelysegmented elements, which may be produced according to the finelysegmented elements described in FIG. 4.

In contrast to the mark of FIG. 2, each of the working zones 102, andmore particularly each of the periodic structures 104 in FIG. 7 has a“rectangular” outline or shape in order to accommodate an additionalstructure 108 in the center of the mark 100. In the illustratedembodiment, the additional structure 108 represents a standard box inbox structure as described in the background of this application. Assuch, the X configured mark 100 can be acquired and imaged by standardbox in box overlay metrology tools and algorithms. It should be noted,however, that this is not a limitation and that the additional structuremay represent other structures, as for example, a pattern recognitionstructure, which can be recognized and acquired by optical patternrecognition tools and algorithms. Both configurations have the advantagethat no change is necessary to existing equipment and software.

FIG. 8 is a top plan view of an overlay mark 110, in accordance with analternate embodiment of the present invention. By way of example,overlay mark 110 may correspond to the mark of FIG. 2. It should benoted, however, that unlike the mark of FIG. 2, the overlay mark 110 isconfigured to measure overlay in two separate directions. As such, mark110 obviates the need to have one mark for each direction in whichoverlay needs to be measured. Overlay mark 110 is shown in aconfiguration that results when the tested layers of a wafer are inperfect alignment.

The overlay mark 110 includes a plurality of working zones 112 fordetermining the registration error between two wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark110 includes four square shaped working zones 112, which are configuredto substantially fill a field of view (not shown) of the metrology toolused to image the overlay mark 110. The working zones 112 represent theactual areas of the mark that are used to calculate alignment betweendifferent layers of the wafer. As mentioned previously, the workingzones 112 are spatially separated from one another so that they do notoverlap portions of an adjacent working zone of the second layer.

In this embodiment, the working zones are configured to provide overlayinformation in two directions, as for example, in the X and Ydirections. Of the four working zones 112A–D, two of them 112A and D aredisposed in the first layer and two of them 112 B and C are disposed inthe second layer (the first layer is represented by solid fill, thesecond layer is represented by no fill). Working zones 112A and D, whichare disposed on the same first layer, are positioned opposite oneanother at a first vertical angle while working zones 112B and 112C,which are disposed on the same second layer, are positioned opposite oneanother at a second vertical angle (e.g., diagonally). Thesecross-positioned structures form an “X” shaped pattern.

Each of the working zones 112 contains an individual periodic structure114, as for example, periodic structures 114A–D. As shown, each of theperiodic structures 114 substantially fills the perimeter of itscorresponding working zone 112. As should be appreciated, each of theperiodic structures 114 are formed in the layer of its correspondingworking zone 112. The periodic structures 114 include coarsely segmentedelements 116 that are arranged in spaced apart rows and columns. Each ofthe coarsely segmented elements 116, in turn, are formed by finelysegmented elements 118. The finely segmented elements 118 are alsoarranged in spaced apart rows and columns. The individual coarselysegmented elements 116 and finely segmented elements 118 may beconfigured with a variety of sizes, shapes and distributions. In theillustrated embodiment, both the coarsely segmented elements 116 andfinely segmented elements 118 are square shaped and equally spaced froman adjacent element. As should be appreciated, overlay mark 110 can beused to measure the misregistration value in two separate directionsthat are perpendicular to each other since the mark 110 has the samerepeating structural pattern in orthogonal directions.

FIG. 9 is a top plan view of an overlay mark 130, in accordance with analternate embodiment of the invention. By way of example, overlay mark130 may correspond to the mark of FIG. 2. It should be noted, however,that unlike the mark of FIG. 2, the overlay mark 130 of FIG. 8 isconfigured to measure overlay in two separate directions. As such, mark130 obviates the need to have one mark for each direction in whichoverlay needs to be measured. Overlay mark 130 is shown in aconfiguration that results when the tested layers of a wafer are inperfect alignment. The overlay mark 130 is generally provided todetermine the relative shift between two or more successive layers of awafer or between two or more separately generated patterns on a singlelayer of a wafer. For ease of discussion, the overlay mark 130 will bedescribed in context of measuring overlay between different layers of asubstrate. It should be noted, however, that the overlay mark in thisfigure may also be used to measure two or more separately generatedpatterns on a single layer of a substrate.

The overlay mark 130 includes a plurality of working zones 132 fordetermining the registration error between two wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark130 includes eight rectangularly shaped working zones 132, which areconfigured to substantially fill its perimeter 71. The working zones 132represent the actual areas of the mark that are used to calculatealignment between different layers of the wafer. As mentionedpreviously, the working zones 132 are spatially separated from oneanother so that they do not overlap portions of an adjacent workingzone. In this particular configuration, some of the working zones areseparated via exclusion zones while other working zones are positionednext to an adjacent working zone. For example, working zone 132B isseparated from working zones 132 E and F via an exclusion zone 133 whileworking zones 132E and F are positioned next to one another at theiredges.

To facilitate discussion, the working zones 132 are grouped into a firstworking group 134 and a second working group 136. The first workinggroup 134 includes four working zones 132A–D that are configured toprovide overlay information in a first direction. By way of example, thefirst direction may be the Y direction. Of the four working zones132A–D, two of them 132A and D are disposed in the first layer and twoof them 132 B and C are disposed in the second layer (the first layer isrepresented by cross hatching, the second layer is represented by nocross hatching). As should be appreciated, for this mark configurationand in the case of zero overlay error (as shown), the centers ofsymmetry 135 of working zones 132A&D and working zones 132B&C coincideexactly. The second working group 136 includes four working zones 132E–Hconfigured to provide overlay information in a second direction that isperpendicular to the first direction. By way of example, the seconddirection may be the X direction. Of the four working zones 132E–H, twoof them 132E and H are disposed in the first layer and two of them 132 Fand G are disposed in the second layer (the first layer is representedby cross hatching, the second layer is represented by no crosshatching). Similarly to the above, for this mark configuration and inthe case of zero overlay (as shown), the centers of symmetry 137 ofworking zones 132E&H and working zones 132F&G coincide exactly.

As should be appreciated, each of the groups 134 and 136 represents an“X”-configured mark (albeit offset). For example, working group 134includes working zones 132A&D, which are on the same first layer and indiagonally opposed positions relative to one another, and working zones132B&C, which are on the same second layer and in diagonally opposedpositions relative to one another. Further, working zones 132A&D areangled relative to working zones 132B&C. Further still, working zone132A is spatially offset from working zone 132D, and working zone 132Bis spatially offset from working zone 132D.

In addition, working group 136 includes working zones 132E&H, which areon the same first layer and in diagonally opposed positions relative toone another, and working zones 132F&G, which are on the same secondlayer and in diagonally opposed positions relative to one another.Further, working zones 132E&H are angled relative to working zones132F&G. Further still, working zone 132E is spatially offset fromworking zone 132H, and working zone 132F is spatially offset fromworking zone 132G. In essence, this particular mark produces two “X”configured marks that are positioned orthogonal to each other, i.e.,working group 134 and working group 136.

To elaborate further, a working zone on one layer is generallyjuxtaposed relative to a working zone on another layer. For example, inthe first working group, working zone 132A is juxtaposed relative toworking zone 132B and working zone 132C is juxtaposed relative toworking zone 132D. Similarly, in the second working group, working zone132E is juxtaposed relative to working zone 132F and working zone 132Gis juxtaposed relative to working zone 132H. Of the two juxtaposedpairs, the working zone on the second layer is typically positionedcloser to the center of the FOV than the working zone on the firstlayer. For example, working zones 132B and C and working zones 132 F andG are positioned closer to the center 142 of the FOV 144 than theirjuxtaposed working zones 132A and D and working zones 132 E and H,respectively. Furthermore, within each of the working groups, thejuxtaposed pairs are positioned in an opposed relationship (e.g.,diagonal) relative to the other juxtaposed pair in the group. Forexample, juxtaposed pairs 132A&B are positioned opposite juxtaposedpairs 132C&D, and juxtaposed pairs 132E&F are positioned oppositejuxtaposed pairs 132G&H.

As should be appreciated, in this particular mark, the configuration ofthe working zones is rotationally symmetric (±90, 180, 270, 360 degreesaround the center of the mark). This is typically done to reduce theimpact of radial and axial variations across the field of view of themetrology tool, as for example, radial and axial variations caused bynon-uniform optical aberrations and illumination that may cause toolinduced shifts (TIS). Radial variations generally refer to variationsthat radiate from the center of the mark to the outer regions of themark. Axial variations generally refer to variations that occur indirections along the axis of the mark, as for example, in the Xdirection from the left to the right portions of the mark, and in the Ydirection from the lower to the upper portions of the mark.

Each of the working zones 132A–H includes a periodic structure 138comprised by a plurality of coarsely segmented lines 140. Thelinewidths, D, and spacings, s, of the coarsely segmented lines may bewidely varied. As shown, each of the periodic structures 138substantially fills the perimeter of its corresponding working zone 132.As should be appreciated, the periodic structures 138 are also disposedon the layer of its corresponding work zone 132.

For ease of discussion, the periodic structures 138 may be broken upinto a first periodic structure 138A that is associated with the firstworking group 134 and a second periodic structure 138B that isassociated with the second working group. As shown, the first periodicstructures 138A are all oriented in the same direction, i.e., thecoarsely segmented lines 140 are parallel and horizontally positionedrelative to each other. The second periodic structures 138B are also alloriented in the same direction (albeit differently than the firstperiodic structures), i.e., the coarsely segmented lines 140 areparallel and vertically positioned relative to each other. As such, theperiodic structures 138A in the first working group 134 are orthogonalto the periodic structures 138B in the second working group 136.

In one embodiment, the coarsely segmented lines of juxtaposed periodicstructures are aligned with one another, i.e., if we ignore thedifferent layers they appear to be continuous gratings. For example, thecoarsely segmented lines of working zone 132A may align with thecoarsely segmented lines of working zone 132B and coarsely segmentedlines of working zone 132C may align with the coarsely segmented linesof working zone 132D. In addition, the coarsely segmented lines ofworking zone 132E may align with the coarsely segmented lines of workingzone 132F and coarsely segmented lines of working zone 132G may alignwith the coarsely segmented lines of working zone 132H.

FIG. 10 is a top plan view of the overlay mark 150, in accordance withan alternate embodiment of the invention. In this particular embodiment,the coarsely segmented lines 140 are formed by a plurality of finelysegmented elements 152. The finely segmented elements 152 generallycorrespond to the finely segmented elements 178 described in FIG. 2.

FIG. 11 is a top plan view of an overlay mark 170, in accordance with analternate embodiment of the present invention. By way of example, theoverlay mark 170 may generally correspond to the overlay mark shown inFIGS. 6 & 9. Similarly to the overlay mark 130, overlay mark 170contains eight working zones 172A–H for determining the registrationerror between two wafer layers in two different directions (one layer isrepresented by cross-hatching, the other is not). Each of the workingzones includes a periodic structure 174 comprised by a plurality ofcoarsely segmented lines 176. Similarly to the overlay mark 90, each ofthe working zones 172, are configured to accommodate an additionalstructure 178 in the center of the mark 170. In the illustratedembodiment, the working zones 172A–H are disposed around the outerregion of the mark while the additional structure 178 is disposed in thecenter of the mark. The additional structure 178 may represent astandard box in box structure as described in the background of thisapplication. As such, the mark 170 can be acquired and measured bystandard box in box metrology tools and algorithms, i.e., existingequipment and software may be used.

FIG. 12 is a top plan view of an overlay mark 190, in accordance with analternate embodiment of the present invention. By way of example,overlay mark 190 may correspond to the mark of FIG. 9. Like the mark ofFIG. 9, the overlay mark 190 of FIG. 12 is configured to measure overlayin two separate directions. As such, mark 190 obviates the need to haveone mark for each direction in which overlay needs to be measured. Incontrast to the mark of FIG. 9, the mark 190 includes triangularlyshaped working zones. Overlay mark 190 is shown in a configuration thatresults when the tested layers of a wafer are in perfect alignment.

The overlay mark 190 includes a plurality of working zones 192 fordetermining the registration error between two wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark140 includes eight triangularly shaped working zones 192, which areconfigured to substantially fill its perimeter. The working zones 192represent the actual areas of the mark that are used to calculatealignment between different layers of the wafer. As mentionedpreviously, the working zones 192 are spatially separated from oneanother so that they do not overlap portions of an adjacent workingzone. In this particular configuration, all of the working zones 192 areseparated via exclusion zones 193.

To facilitate discussion, the working zones 192 are grouped into a firstworking group 194 and a second working group 196. The first workinggroup 194 includes four working zones 192A–D that are configured toprovide overlay information in a first direction. By way of example, thefirst direction may be the Y direction. Of the four working zones192A–D, two of them 192A and D are disposed in the first layer and twoof them 192 B and C are disposed in the second layer (the first layer isrepresented by solid lines, the second layer is represented by dashedlines). As should be appreciated, for this mark configuration and in thecase of zero overlay (as shown), the centers of symmetry 195 of workingzones 192A&D and working zones 192B&C coincide exactly. The secondworking group 196 includes four working zones 192E–H configured toprovide overlay information in a second direction that is perpendicularto the first direction. By way of example, the second direction may bethe X direction. Of the four working zones 192E–H, two of them 192 E andH are disposed in the first layer and two of them 192 F and G aredisposed in the second layer (the first layer is represented by solidlines, the second layer is represented by dashed lines). Similarly tothe above, for this mark configuration and in the case of zero overlay(as shown), the centers of symmetry 197 of working zones 192E&H andworking zones 192F&G coincide exactly. In addition, and all of theworking zones 192 are equally positioned relative to the center of themark.

As should be appreciated, each of the groups 194 and 196 represents an“X”-configured mark. For example, working group 194 includes workingzones 192A&D, which are on the same first layer and in diagonallyopposed positions relative to one another, and working zones 192B&C,which are on the same second layer and in diagonally opposed positionsrelative to one another. Further, working zones 192A&D are angledrelative to working zones 192B&C. Further still, working zone 192A isspatially offset from working zone 192D, and working zone 192B isspatially offset from working zone 192D.

In addition, working group 196 includes working zones 192E&H, which areon the same first layer and in opposed positions relative to oneanother, and working zones 192F&G, which are on the same second layerand in opposed positions relative to one another. Further, working zones192E&H are angled relative to working zones 192F&G. Further still,working zone 192E is spatially offset from working zone 192H, andworking zone 192F is spatially offset from working zone 192G. Inessence, this particular mark produces two “X” configured marks that arepositioned orthogonal to each other, i.e., working group 194 and workinggroup 196.

To elaborate further, a working zone on one layer is generallyjuxtaposed relative to a working zone on another layer. For example, inthe first working group, working zone 192A is juxtaposed relative toworking zone 192B and working zone 192C is juxtaposed relative toworking zone 192D. Similarly, in the second working group, working zone192E is juxtaposed relative to working zone 192F and working zone 192Gis juxtaposed relative to working zone 192H. For this mark configurationand in the case of zero overlay (as shown), all of the working zones 192are equally positioned relative to the center of the mark. Furthermore,within each of the working groups, the juxtaposed pairs are positionedin an opposed relationship (e.g., upper/lower and right/left) relativeto the other juxtaposed pair in the group. For example, juxtaposed pairs192A&B are positioned opposite juxtaposed pairs 192C&D, and juxtaposedpairs 192E&F are positioned opposite juxtaposed pairs 192G&H.

As should be appreciated, in this particular mark, the configuration ofthe working zones is rotationally symmetric (±90, 180, 270, 360 degreesaround the center of the mark) without biasing the center or peripherywith one or other layer, i.e., the mark is invariant. This is typicallydone to reduce the impact of radial and axial variations across thefield of view of the metrology tool, as for example, radial and axialvariations caused by optical aberrations and illuminations that maycause tool induced shifts (TIS).

Each of the working zones 192 includes a periodic structure 198comprised by a plurality of coarsely segmented lines 200. Thelinewidths, D, and spacings, s, of the coarsely segmented lines may bewidely varied. As shown, each of the periodic structures 198substantially fills the perimeter of its corresponding working zone 192.As should be appreciated, the periodic structures 198 are also disposedon the layer of its corresponding work zone 192.

For ease of discussion, the periodic structures 198 may be broken upinto a first periodic structure 198A that is associated with the firstworking group 194 and a second periodic structure 198B that isassociated with the second working group 196. As shown, the firstperiodic structures 198A are all oriented in the same direction, i.e.,the coarsely segmented lines are parallel and horizontally positionedrelative to each other. The second periodic structures 198B are also alloriented in the same direction (albeit differently than the firstperiodic structures), i.e., the coarsely segmented lines 198B areparallel and vertically positioned relative to each other. As such, theperiodic structures 198A in the first working group 194 are orthogonalto the periodic structures 198B in the second working group 196.Furthermore, in order to accommodate each zone within the FOV, thecoarsely segmented lines 190 decrease in length as they move from theouter regions of the mark to the inner regions of the mark. Although notshown, the coarsely segmented line may be formed by a plurality offinely segmented elements to further enhance this mark.

FIG. 13 is a top plan view of an overlay mark 210, in accordance with analternate embodiment of the present invention. As shown, mark 210 hasthe same general layout and characteristics as mark 190 of FIG. 12,i.e., eight triangularly shaped working zones. Mark 210 differs frommark 190, however, in that it biases the center of the mark with agrating pattern 212 formed on one of the two layers. The grating pattern212 is typically used in cases where the mark quality in one layer ispoorer than the mark quality in the other layer due to contrast orgraininess. That is, the information (e.g., edges) in a layer wherecontrast is low is increased. Alternatively, biasing the center of theFOV with one layer may protect it from process damage. The gratingpattern 212 may be widely varied. For example, grating pattern mayinclude any number of lines in any number of distributions and sizes. Inthis particular embodiment, the grating pattern is formed on the secondlayer and it consists of groups of two coarsely segmented lines 214 thatalternate in direction (e.g., X and Y directions) around the center ofthe mark.

FIG. 14 is a top plan view of an overlay mark 220, in accordance with analternate embodiment of the present invention. By way of example,overlay mark 220 may generally correspond to the overlay mark shown inFIG. 9. Like the overlay mark of FIG. 9, overlay mark 220 is configuredto measure overlay in two separate directions. As such, mark 220obviates the need to have one mark for each direction in which overlayneeds to be measured. Overlay mark 220 is shown in a configuration thatresults when the tested layers of a wafer are in perfect alignment.

The overlay mark 220 includes a plurality of working zones 222 fordetermining the registration error between two wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark222 includes sixteen square shaped working zones 222, which areconfigured to substantially fill its perimeter. Each of the workingzones 222 includes a periodic structure of coarsely segmented lines.Although not shown, it should be appreciated that in some embodimentsthe coarsely segmented lines may be formed by a plurality of finelysegmented elements, as for example, any configuration described or shownherein (i.e., FIGS. 3–5).

Of the 16 working zones, 8 of the working zones 222A are oriented in theX direction and 8 of the working zones 222B are oriented in the Ydirection (as shown by the periodic structures disposed therein). Of the8 working zones 222, in any given orientation (A or B), 4 of the workingzones 222′ are printed in a first layer (represented by cross hatching)while 4 of the working zones 222″ are printed in a second layer (notrepresented by cross hatching). The orientation of the working zones maybe described in a variety of ways. For example, the working zones 222may be distributed into four groups 224A–D that form the four corners ofthe square shaped mark 220. Each of these groups 224A–D is equallyrepresented by working zones formed on different layers and in differentdirection. That is, each group includes four different working zones, asfor example, working zones 222A′, 222A″, 222B′ and 222B″.

The working zones 222 may also be distributed into four groups 224E–H,each of which represents an “X”-configured mark (albeit offset). In thiscase, the “X”-configured mark is formed by the corners of a 3 by 3working zone group. Of the four groups, two of them determine overlay inthe X-direction and two of them determine overlay in the Y direction.For example, working group 224E and F, which include diagonally opposedand spatially offset working zones 222B′& 222B″, determine overlay inthe Y-direction. Furthermore, working group 224G and H, which includediagonally opposed and spatially offset working zones 222A′& 222A″,determine overlay in the X-direction.

FIG. 15 is a top plan view of an overlay mark 240, in accordance with analternate embodiment of the present invention. By way of example,overlay mark 240 may generally correspond to the overlay mark shown inFIG. 14. Like the overlay mark of FIG. 13, overlay mark 240 isconfigured to measure overlay in two separate directions. As such, mark240 obviates the need to have one mark for each direction in whichoverlay needs to be measured. Overlay mark 240 is shown in aconfiguration that results when the tested layers of a wafer are inperfect alignment.

The overlay mark 240 includes a plurality of working zones 242 fordetermining the registration error between two wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark240 includes sixteen square shaped working zones 242, which areconfigured to substantially fill its perimeter. Each of the workingzones 242 includes a periodic structure of coarsely segmented lines.Although not shown, it should be appreciated that in some embodimentsthe coarsely segmented lines may be formed by a plurality of finelysegmented elements.

Of the 16 working zones, 8 of the working zones 242A are oriented in theX direction and 8 of the working zones 242B are oriented in the Ydirection (as shown by the periodic structures disposed therein). Of the8 working zones 242, in any given orientation (A or B), 4 of the workingzones 242′ are printed in a first layer (represented by cross hatching)while 4 of the working zones 242″ are printed in a second layer (notrepresented by cross hatching). The orientation of the working zones maybe described in a variety of ways. For example, the working zones 242may be distributed into four groups 244A–D that form the four corners ofthe square shaped mark 240. The groups that are opposed at verticalangles are identical, i.e., the working zones therein are oriented thesame way. Of the four working zones in each group 244A–D, two of themrepresent the same layer and direction, and two of them represent adifferent same layer and direction. The working zones that are opposedat vertical angles to one another in these groups are identical, i.e.,they represent the same layer and direction. For example, groups 244A &D include opposed working zones 242A′ and opposed working zones 242B″,and groups 244B & C include opposed working zones 242B′ and opposedworking zones 242A″.

The working zones 242 may also be distributed into four groups 244E–H,each of which represents an “X”-configured mark (albeit offset). In thiscase, the “X”-configured mark is formed by the corners of a 3 by 3working zone group. Of the four groups, two of them determine overlay inthe X-direction and two of them determine overlay in the Y direction.For example, working group 244E and F, which include opposing workingzones 242B′& 242B″, determine overlay in the Y-direction. Furthermore,working group 244G and H, which include opposing working zones 242A′&242A″, determine overlay in the X-direction.

FIG. 16 is a top plan view of an overlay mark 250, in accordance with analternate embodiment of the present invention. By way of example,overlay mark 250 may generally correspond to the overlay mark shown inFIG. 13. Like the overlay mark of FIG. 14, overlay mark 250 isconfigured to measure overlay in two separate directions. As such, mark250 obviates the need to have one mark for each direction in whichoverlay needs to be measured. Overlay mark 250 is shown in aconfiguration that results when the tested layers of a wafer are inperfect alignment.

The overlay mark 250 includes a plurality of working zones 252 fordetermining the registration error between two wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark250 includes sixteen square shaped working zones 252, which areconfigured to substantially fill its perimeter. Each of the workingzones 252 includes a periodic structure of coarsely segmented lines.Although not shown, it should be appreciated that in some embodimentsthe coarsely segmented lines may be formed by finely segmented elements.

Of the 16 working zones, 8 of the working zones 252A are oriented in theX direction and 8 of the working zones 252B are oriented in the Ydirection (as shown by the periodic structures disposed therein). Of the8 working zones 252, in any given orientation (A or B), 4 of the workingzones 252′ are printed in a first layer (represented by cross hatching)while 4 of the working zones 252″ are printed in a second layer (notrepresented by cross hatching). As shown, the working zones 252 may bedistributed into four groups 254E–H, each of which represents an“X”-configured mark (albeit offset). In this case, the “X”-configuredmark is formed by the corners of a 3 by 3 working zone group. Of thefour groups, two of them determine overlay in the X-direction and two ofthem determine overlay in the Y direction. For example, working group254E and F, which include opposing working zones 252B′& 252B″, determineoverlay in the Y-direction. Furthermore, working group 254G and H, whichinclude opposing working zones 252A′& 252A″, determine overlay in theX-direction.

FIG. 17 is a top plan view of an overlay mark 270, in accordance with analternate embodiment of the present invention. By way of example,overlay mark 270 may generally correspond to the overlay mark shown inFIG. 13. Like the overlay mark of FIG. 14, overlay mark 270 isconfigured to measure overlay in two separate directions. As such, mark270 obviates the need to have one mark for each direction in whichoverlay needs to be measured. Overlay mark 270 is shown in aconfiguration that results when the tested layers of a wafer are inperfect alignment.

The overlay mark 270 includes a plurality of working zones 272 fordetermining the registration error between two wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark270 includes sixteen square shaped working zones 272, which areconfigured to substantially fill its perimeter. Each of the workingzones 272 includes a periodic structure of coarsely segmented lines.Although not shown, it should be appreciated that in some embodimentsthe coarsely segmented lines may be formed by finely segmented elements.

Of the 16 working zones, 8 of the working zones 272A are oriented in theX direction and 8 of the working zones 272B are oriented in the Ydirection (as shown by the periodic structures disposed therein). Of the8 working zones 272, in any given orientation (A or B), 4 of the workingzones 272′ are printed in a first layer (represented by cross hatching)while 4 of the working zones 272″ are printed in a second layer (notrepresented by cross hatching). Furthermore, of the 8 working zones 272,in any given orientation (A or B), 4 of the working zones 272 have aperiodic structure M with a first period (represented by thinner lines)while 4 of the working zones 272 have a periodic structure N with asecond period that is different than the first period (represented bywider lines).

The orientation of the working zones may be described in a variety ofways. For example, the working zones 272 may be distributed into fourgroups 274A–D that form the four corners of the square shaped mark 270.The groups that are opposed at vertical angles are identical, i.e., theworking zones therein are oriented the same way. Each of these groups274A–D includes four different working zones, which may representdifferent combinations of layers, directions or period. The workingzones that are opposed at vertical angles to one another in these groupsrepresent the same direction, but different layers and periods. Forexample, groups 272A & D include working zone 272A-M′ that is opposed toworking zone 272B-N′ and working zone 272A-M″ that is opposed to workingzone 272B-N″, and groups 272B & C include working zone 272B-M′ that isopposed to working zone 272A-N′ and working zone 272B-M″ is opposed toworking zone 272A-N″.

The working zones 272 may also be distributed into four groups 274E–H,each of which represents an “X”-configured mark (albeit offset). In thiscase, the “X”-configured mark is formed by the corners of a 3 by 3working zone group. Of the four groups, two of them determine overlay inthe X-direction and two of them determine overlay in the Y direction.Furthermore, two of them represent a first period and two of themrepresent a second period. For example, working group 274E, whichincludes opposing working zones 272B-M′& 272B-M″, has a first periodthat determines overlay in the Y-direction, and working group 274F,which includes opposing working zones 272B-N′& 272B-N″, has a secondperiod that determines overlay in the Y-direction. Furthermore, workinggroup 274G, which include opposing working zones 272A-M′& 272A-M″, has afirst period that determines overlay in the X-direction, and workinggroup 274H, which include opposing working zones 272AN′& 272A-N″, has asecond period that determines overlay in the X-direction.

As should be appreciated, this configuration can result in improvedprocess robustness and/or improved contrast for one of the two gratingperiods, allowing selection of the optimized period and or line widthfor a specific process.

FIG. 18 is a top plan view of an overlay mark 290, in accordance with analternate embodiment of the present invention. By way of example,overlay mark 290 may generally correspond to the overlay mark shown inFIG. 13. Like the overlay mark of FIG. 14, overlay mark 290 isconfigured to measure overlay in two separate directions. As such, mark290 obviates the need to have one mark for each direction in whichoverlay needs to be measured. Unlike the overlay mark of FIG. 13,overlay mark 290 is also configured to determine the relative shiftbetween three successive layers of a wafer or between three separatelygenerated patterns on a single layer of a wafer. For ease of discussion,the overlay mark 290 will be described in context of measuring overlaybetween different layers of a substrate. It should be noted, however,that the overlay mark in this figure may also be used to measure two ormore separately generated patterns on a single layer of a substrate.Overlay mark 290 is shown in a configuration that results when thetested layers of a wafer are in perfect alignment.

The overlay mark 290 includes a plurality of working zones 292 fordetermining the registration error between three wafer layers in twodifferent directions. In the illustrated embodiment, the overlay mark290 includes sixteen square shaped working zones 292, which areconfigured to substantially fill its perimeter 7. Each of the workingzones 292 includes a periodic structure of coarsely segmented lines.Although not shown, it should be appreciated that in some embodimentsthe coarsely segmented lines may be formed by finely segmented elements.

Of the 16 working zones 292, 8 of the working zones 292A are oriented inthe X direction and 8 of the working zones 292B are oriented in the Ydirection (as shown by the periodic structures disposed therein).Furthermore, 8 of the working zones 292′ are printed in a first layer(represented by cross hatching), 4 of the working zones 292″ are printedin a second layer (not represented by cross hatching), and 4 of theworking zones 292′″ are printed in a third layer (represented by fill).In this particular embodiment, the first layer (also represented by asingle prime) is disposed over the second layer (also represented by adouble prime) and the second layer is disposed over the third layer(also represented by a triple prime). By way of example, the first layermay represent a resist layer, the second layer may represent a firstmetal layer, and the third layer may represent a second metal layer.

It should be noted that the above configuration may be widely varied.For example, of the 8 working zones in any given orientation (A or B), 2may be printed in a first layer, while each additional pair of gratingsmay be printed in up to any of 3 previous layers.

All of the overlay marks described above are configured to at leastbalance non-uniformities caused by the metrology tool (e.g., aberrationsand illumination) and/or by the process (e.g., dishing and erosion). Forexample, some of the marks may be configured to reduce the impact ofradial variations while others may be configured to reduce the impact ofaxial variations on the overlay measurements.

FIG. 19 is a simplified diagram of an overlay measurement system ormetrology tool 320 that may be used to measure overlay in any of themarks described above via imaging. Imaging is a very developedtechnology with large user acceptance, and components that are readilyavailable to the user. As is generally well known, imaging is aneffective way to collect a large amount of information at any one time.That is, all points within the mark may be measured simultaneously.Furthermore, imaging allows a user to see what is actually beingmeasured on the wafer. The dimensions of various components areexaggerated to better illustrate this embodiment. The overlaymeasurement system 320 is arranged to determine overlay error via one ormore overlay targets 322 disposed on a wafer 324. In most cases, theoverlay targets 322 are positioned within the scribe lines of the wafer324. As is generally well known, scribe lines are the areas of the waferused for sawing and dicing the wafer into a plurality of dies. It shouldbe noted, however, that this is not a limitation and that the positionof the targets may vary according to the specific needs of each devicedesign. As shown, the overlay measurement system 320 includes an opticalassembly 326 and a computer 328. The optical assembly 326 is generallyarranged to capture the images of the overlay target 322. The computer,on the other hand, is generally arranged to calculate the relativedisplacement of the elements of the overlay target from the capturedimages.

In the illustrated embodiment, the optical assembly 326 includes a lightsource 330 (e.g., incoherent or coherent, although incoherent isgenerally preferred) arranged to emit light 332 along a first path 334.The light 332 is made incident on a first lens 335, which focuses thelight 332 onto a fiber optic line 336 configured to pass the light 332therethrough. When the light 332 emerges from fiber optic line 336, itthen passes through a second lens 338, which is arranged to collimatethe light 332. The collimated light 332 then continues on its path untilit reaches a beam splitter cube 340, which is arranged to direct thecollimated light onto a path 342. The collimated light 332 continuingalong path 342 is made incident on an objective lens 344, which focusesthe light 332 onto the wafer 324.

The light 332, which reflects off of the wafer 324, is then collected bythe objective lens 344. As should be appreciated, the reflected light332 that is collected by the objective lens 344 generally contains animage of a portion of the wafer 324, as for example, the image of theoverlay target 322. When the light 332 leaves the objective 344, itcontinues along path 342 (backwards) until it reaches the beam splittercube 340. In general, the objective lens 344 manipulates the collectedlight in a manner that is optically reverse in relation to how theincident light was manipulated. That is, the objective lens 344re-collimates the light 332 and directs the light 332 towards the beamsplitter cube 340. The beam splitter cube 340 is arranged to direct thelight 332 onto a path 346. The light 332 continuing on path 346 is thencollected by a tube lens 350, which focuses the light 332 onto a camera352 that records the image of the wafer 324, and more particularly theimage of the target 322. By way of example, the camera 352 may be acharge couple device (CCD), a two-dimensional CCD, or linear CCD array.In most cases, the camera 352 transforms the recorded image intoelectrical signals, which can be used by, and which are sent to thecomputer 328. After receiving the electrical signals, the computer 328performs analysis algorithms that calculate the overlay error of theimage. Analysis algorithms will be described in greater detail below.

The system 320 further includes a frame grabber 354 that works with thecomputer 328 and the camera 352 to grab images from the wafer 324.Although the frame grabber 354 is shown as a separate component, itshould be noted that the frame grabber 354 may be part of the computer328 and/or part of the camera 352. The frame grabber 354 typically hastwo functions—target acquisition and image grab. During targetacquisition, the frame grabber 354 and computer 328 cooperate with awafer stage 356 to place the target in focus and to position the targetas closes as possible to the center of the field of view (FOV) of themetrology tool. In most cases, the frame grabber grabs a plurality ofimages (e.g., not the images used to measure overlay) and the stagemoves the wafer between these grabs until the target is correctlypositioned in the X, Y and Z directions. As should be appreciated, theX&Y directions generally correspond to the field of view (FOV) while theZ direction generally corresponds to the focus. Once the frame grabberdetermines the correct position of the target, the second of these twofunctions is implemented (e.g., image grab). During image grab, theframe grabber 354 makes a final grab or grabs so as to capture and storethe correctly positioned target images, i.e., the images that are usedto determine overlay.

After grabbing the images, information must be extracted from thegrabbed images to determine the registration error. The extractedinformation may be digital information or in waveforms. Variousalgorithms may then be used to determine the registration error betweenvarious layers of a semiconductor wafer. For example, a frequency domainbased approach, a space domain based approach, Fourier transformalgorithms, zero-crossing detection, correlation and cross-correlationalgorithms and others may be used.

Algorithms proposed for determining overlay via the marks describedherein (e.g., marks that contain periodic structures) can generally bedivided into a few groups. For instance, one group may relate to phaseretrieval based analysis. Phase retrieval based analysis, which is oftenreferred to as frequency domain based approaches, typically involvescreating one dimensional signals by collapsing each of the working zonesby summing pixels along the lines of the periodic structure. Examples ofphase retrieval algorithms that may be used are described in U.S. Pat.No. 6,023,338 issued to Bareket, U.S. patent application Ser. No.09/603,120 filed on Jun. 22, 2000, and U.S. patent application Ser. No.09/654,318 filed on Sep. 1, 2000, all of which are incorporated hereinby reference.

Yet another phase retrieval algorithm that may be used is described inU.S. application Ser. No. 09/697,025 filed on Oct. 26, 2000, which isalso incorporated herein by reference. The phase retrieval algorithmdisclosed therein decomposes signals into a set of harmonics of thebasic signal frequency. Quantitative comparison of different harmonics'amplitudes and phases provide important information concerning signals'symmetry and spectral content. In particular, the phase differencebetween the 1st and 2nd or higher harmonics of the same signal(calibrated with their amplitudes) measures the degree of the signalasymmetry. The major contributions to such asymmetry come from theoptical misalignment and illumination asymmetry in the metrology tool(tool induced shifts), as well as process induced structural features(wafer induced shifts). Comparing this misregistration between thephases of the 1st and the 2nd harmonics for the signals acquired fromdifferent parts of the field of view on the same process layer mayprovide independent information about optical aberrations of themetrology tool. Finally, comparing these misregistrations frommeasurements at a given orientation with those obtained after rotatingthe wafer 180 degrees allows separation of the tool induced and waferinduced shifts due to asymmetry.

Yet another phase retrieval algorithm that may be used is Waveletanalysis. Wavelet analysis is somewhat similar to that described in thesection above, however, now a dynamic window is moved across the onedimensional signal and the phase estimation is carried out in a morelocalized way. This is particularly of interest with use in the case ofa chirped periodic structure.

Another group may relate to intensity correlation based methods. In thisapproach the centers of symmetry for each process layer is foundseparately by calculating the cross covariance of one signal with thereversed signal from the opposite part of the mark, from the sameprocess layer. This technique is similar to techniques used today withregards to box in box targets.

The above techniques are brought by way of example and have been testedand demonstrated good performance. Other alternative algorithmic methodsfor calculation of overlay include other variations of auto & crosscorrelation techniques, error correlation techniques, error minimizationtechniques, such as minimization of absolute difference, minimization ofthe square of the difference, threshold based techniques including zerocross detection, and peak detection. There are also dynamic programmingalgorithms which can be used for searching for the optimal matchingbetween two one-dimensional patterns. As mentioned above, the analysisalgorithms and approaches may be utilized with respect to all of thevarious overlay marks described in the previous section.

Importantly, it should be noted that the above diagram and descriptionthereof is not a limitation and that the overlay image system may beembodied in many other forms. For example, it is contemplated that theoverlay measurement tool may be any of a number of suitable and knownimaging or metrology tools arranged for resolving the critical aspectsof overlay marks formed on the surface of the wafer. By way of example,overlay measurement tool may be adapted for bright field imagingmicroscopy, darkfield imaging microscopy, full sky imaging microscopy,phase contrast microscopy, polarization contrast microscopy, andcoherence probe microscopy. It is also contemplated that single andmultiple image methods may be used in order to capture images of thetarget. These methods include, for example, single grab, double grab,single grab coherence probe microscopy (CPM) and double grab CPMmethods. These types of systems, among others, are readily availablecommercially. By way of example, single and multiple image methods maybe readily available from KLA-Tencor of San Jose, Calif.

FIG. 20A is a simplified flow diagram illustrating a method 360 ofcalculating overlay, in accordance with one embodiment of the presentinvention. For ease of discussion, this method will be described via themark shown in FIG. 9. This mark is now shown next to the flow diagram inFIG. 20B. The method 360 begins at step 362 where the working zones areselected from the captured image. By way example, for calculatingX-overlay, working zones 132 E–H may be selected, and for calculatingY-overlay, working zones 132 A–D may be selected. After selecting theworking zones, the process flow proceeds to step 364 whererepresentative signals are formed for each of the selected workingzones. This may be accomplished by collapsing the 2D images into 1Dsignals by averaging over X for Y-overlay calculations and by averagingover Y for X-overlay calculations. By way of example, FIG. 20Cillustrates a first collapsed 1D signal for working zone 132A and asecond collapsed 1D signal for working zone 132B. It should be noted,that FIG. 20C is representative of any of the pairs of juxtaposedworking zones. After forming the signals, the process flow proceeds tostep 366 where the overlay is determined by comparing the signals.

In one embodiment, this is accomplished via a covariance-based overlayalgorithm, which is based upon calculation of the cross-correlationbetween the patterns belonging to the same process layers. As a result,the centers of symmetry for both layers are found, and theirmisregistration is essentially the overlay. The flowchart of thisalgorithm is shown in FIG. 21.

In another embodiment, this is accomplished via a Fourier Decompositionoverlay algorithm, which utilizes the periodical character of thegrating structures. This algorithm decomposes signals acquired from thetarget patterns to a series of Fourier harmonics. Comparison of phasesbetween the same order harmonics from different process layerscalibrated to nominal pitch of the grating patterns serves then as abasis for overlay calculation. Accordingly, this algorithm providesseveral independent overlay results—one for each Fourier order. Theflowchart of this algorithm is shown in FIG. 22.

FIG. 21 is a flow diagram illustrating a method 370 of calculatingoverlay using Covariance, in accordance with one embodiment of thepresent invention. By way of example method 370 may generally correspondto step 366 of FIG. 20A. The method 370 begins at step 372 where thesignal cross-correlation is calculated. This is typically done withrespect to opposing working zones. With regards to the mark of FIG. 9.,the signal cross-correlation is calculated for working zone pairs 132Avs. reversed 132D, 132B vs. reversed 132C, 132E vs. reversed 132H, and132F vs. reversed 132G. After calculating the signal cross correlation,the process flow proceeds to step 374 where the positions ofcross-correlation maxima (sub pixel) are found. This is typically donefor both of the layers, i.e., layer 1, which is represented by crosshatching, and layer 2, which is represented with no cross hatching.After finding the positions of cross correlation maxima, the processflow proceeds to step 376 where the overlay is determined by calculatingthe difference between the positions of cross correlation maxima. Forexample, the difference between the cross correlation maxima of workingzones 132E&H (layer 1)−working zones 132 F&G (layer 2) determines themisregistration in the X-direction. In addition, the difference betweenthe cross correlation maxima of working zones 132A&D (layer 1)−workingzones 132 B&C (layer 2) determines the misregistration in theY-direction.

FIG. 22 is a flow diagram illustrating a method 380 of calculatingoverlay using Fourier Decomposition, in accordance with one embodimentof the present invention. By way of example method 380 may generallycorrespond to step 366 of FIG. 20A. The method 380 begins at step 382where the signals are fitted to a Fourier series and their phases areextracted. After fitting and extracting, the process flow proceeds tostep 384 where the phase difference between juxtaposed working zones isfound. For example, in the Y direction, the phase difference is foundbetween working zones 132A and 132B, as well as, between working zones132C and 132D. In addition, in the X direction, the phase difference isfound between working zones 132E and 132F, as well as, between workingzones 132G and 132H. After finding the phase difference, the processflow proceeds to step 386 where the overlay is determined by calculatingthe difference between the phase differences of the previous step for agiven direction. For example, the average of the difference between thephase difference of working zones 132E&F and the phase difference ofworking zones 132 G&H determines the misregistration in the X-direction.In addition, the average of the difference between the phase differenceof working zones 132A&B and the phase difference of working zones 132C&D determines the misregistration in the Y-direction. In order toobtain data that corresponds to positions of the layers, the phasedifference is multiplied by the pitch and divided by 2π.

In summary, a method of designing overlay marks in accordance with theprinciples set forth above will now be described.

FIG. 23 is a flow diagram illustrating a method 390 of designing anoverlay mark, in accordance with one embodiment of the presentinvention. By way of example, the overlay mark may generally correspondto any of the overlay marks described above. The overlay mark isgenerally provided to determine the relative shift between two or moresuccessive layers of a substrate or between two or more separatelygenerated patterns on a single layer of a substrate. In accordance withone embodiment of the present invention, the overlay mark comprises aplurality of elements that form the pattern of the overlay mark. Each ofthese elements may be configured to contain information for measuringoverlay in a more precise or accurate manner. By way of example, theelements may generally correspond to the working zones, periodicstructures of coarsely segmented elements and finely segmented elements.

The method 390 begins at step 392 where the geometry of a first elementof the mark is optimized according to a first scale. This is typicallyaccomplished by identifying the upper and lower limits of the firstscale and fine tuning the geometry of the first geometry between theupper and lower limits. In one embodiment, the first scale correspondsto the metrology kernel scale, which defines the boundaries of theregions that contain information about the two different layers orpatterns between which overlay is to be measured. The metrology kernelscale has characteristic dimensions in the 10's of microns. For example,the metrology kernel scale may range from about 4 microns to about 10microns, and more particularly from about 5 microns to about 10 microns.

In most cases, the metrology kernel scale is based on metrology toollimitations, process issues and circuit design criteria. Metrology toollimitations generally refers to limitations associated with themetrology tool (both for a line of tools and a specific tool within theline). For example, metrology tool limitations may include the size andshape of the field of view of the metrology tool used to measureoverlay, the minimum amount of target area needed to effectively measureoverlay and asymmetrical aberration and illumination field distributionscreated by the components of the metrology tool. Process robustnessissues generally refer to restrictions associated with the robustness ofthe process, as for example, etching, deposition, chemical-mechanicalpolishing (CMP) and the like. For example, there may be parts of themark that are more susceptible to process damage or process variationand thus they should be avoided. Circuit design criteria generallyrefers to the rules used to design the overall circuit pattern. Forexample, the circuit design rules may include scribe line limitationsthat pertain to the overlay mark budget, i.e., the mark is typicallypositioned inside the scribe line of the wafer. The scribe line is theplace on the wafer where the wafer separated into a plurality of diesvia sawing or dicing.

Once the first scale is identified, the geometry of the first element,which generally contains information relating to the layers or patternsbetween which overlay is measured, may be fine-tuned to find a marklayout that works best within this scale. In one embodiment, the firstelement corresponds to working zones, which define the different layersor patterns of the overlay mark, and which represent the actual area ofthe overlay mark that is used for overlay measurements. The term“geometry” generally refers to the size, shape, and/or distribution ofthe first element, i.e., the working zones. In one embodiment,fine-tuning is implemented by defining the perimeter of the mark (e.g.,FOV) and dividing the mark into a plurality of working zones that areconfigured to minimize the impact of asymmetries on the measurement ofoverlay. By way of example, the working zones may be configured tominimize the impact of optical aberrations and illuminations on toolinduced shifts in the resultant overlay measurement. In addition, theworking zones may be configured to minimize the impact of processvariations on wafer induced shifts in the resultant overlay measurement.

After optimizing the geometry of the first element, the process flowproceeds to step 394 where the geometry of a second element of the markis optimized according to a second scale. This is typically accomplishedby identifying the upper and lower limits of the second scale and finetuning the geometry of the second element between the upper and lowerlimits. In one embodiment, the second scale corresponds to the imageresolution scale, which defines the boundaries between structures withina given process layer. The image resolution scale has characteristicdimensions in the 1 micron range. For example, the image resolutionscale may range from about 0.3 microns to about 2 microns, and moreparticularly from about 0.5 microns to about 1 micron.

In most cases, the image resolution scale is based on metrology toollimitations and process robustness issues. Metrology tool limitationsgenerally refers to limitations associated with the metrology tool (bothfor a line of tools and a specific tool within the line). For example,metrology tool limitations may include the image resolution of the tool,i.e., the ability to capture an image, the algorithms used by the toolto calculate the overlay error and the aberration and illumination fielddistributions of the tool. Process robustness issues generally refer torestrictions that are created by the materials and processes that areused to form the layers and patterns on the wafer, as for example,etching, deposition, chemical-mechanical polishing (CMP) and the like.For example, for specific processes, such as aluminum coated, chemicallymechanically polished tungsten, it is advantageous for the geometry ofthe second element to be one micron or less in order to diminish theimpact of asymmetries resultant from the polishing and aluminumdeposition processes. In other cases, where the metal grain size islarge, it may be preferable that the lines be larger than 1 micron, asfor example, up to two microns.

Once the second scale is identified, the geometry of the second element,which generally contains the actual spatial information regarding therelative positions of the mark components that is encoded andtransferred to the metrology tool, may be fine-tuned to find a marklayout that works best within this scale. In one embodiment, the secondelement corresponds to a periodic structure of coarsely segmented linesthat is positioned within each of the working zones of the firstelement. The term “geometry” generally refers to the size, shape, and/ordistribution of the second element i.e., the periodic structure ofcoarsely segmented lines (e.g., linewidths and spacings). In oneembodiment, the periodic structures via the coarsely segmented lines areconfigured to enhance the measurement of overlay by balancing the imageresolution of the tool with the process.

After optimizing the geometry of the second element, the process flowproceeds to step 396 where the geometry of a third element of the markis optimized according to a third scale. This is typically accomplishedby identifying the upper and lower limits of the third scale and finetuning the geometry of the third element between the upper and lowerlimits. In one embodiment, the third scale corresponds to thelithography resolution scale, which defines the boundaries ofsub-structures within a given structure. The lithography resolutionscale has characteristic dimensions in the 0.1 micron range. Forexample, the lithography resolution scale may range from about 0.01microns to about 0.5 microns, and more particularly from about 0.05microns to about 0.18 microns.

In most cases, the lithography resolution scale is based on circuitdesign rules, process robustness issues and metrology tool limitations.Circuit design rules generally refer to the rules used to design theoverall circuit pattern. For example, the circuit design rules mayinclude the geometry of the circuit devices (e.g., feature size anddensity). Process robustness issues generally refer to restrictions thatare created by the materials and processes that are used to form thelayers and patterns on the wafer, as for example, etching, deposition,chemical-mechanical polishing (CMP) and the like. Metrology toollimitations generally refer to limitations associated with the metrologytool (both for a line of tools and a specific tool within the line). Forexample, metrology tool limitations may include contrast requirements ofthe tool, i.e., the ability to resolve the larger structures, which arecomprised by the smaller sub-structures.

Once the third scale is identified, the geometry of the third element,which generally contains information reflective of the circuitstructures themselves, may be fine-tuned to find a mark layout thatworks best within this scale. In one embodiment, the third elementcorresponds to the finely segmented elements that form the plurality ofcoarsely segmented lines of the periodic structures of the secondelement. The term “geometry” generally refers to the size, shape, and/ordistribution of the third element, i.e., the finely segmented elements.In one embodiment, the finely segmented elements are configured tominimize the adverse effects of the process so as to improve theaccuracy and precision of the overlay measurements. In anotherembodiment, the structure is optimized so as to minimize the differencein stepper PPE between the target and the actual device, as describedpreviously in FIGS. 3–5.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of this invention.

For example, although the invention has been described in terms ofmanufacturing semiconductor devices, it should be realized that theinvention may also be suitable for manufacturing other types of devicessuch as microfabrication of optical or optoelectronic devices,microfabrication of magnetic storage media or magnetic storageread/write or input/output devices, microfabrication using lithographicpatterning in general, to include photolithography down to 100 nmexposure wavelengths, extreme-ultraviolet lithography with wavelengths10 nm to 100 nm, Xrays lithography with wavelengths <10 nm, electronbeam lithography, ion beam patterning, or mixed lithography using morethan one of these methods.

In addition, besides supplying data for overlay measurements, periodicstructure targets are capable of providing much additional informationfor target, stepper and metrology tool diagnostics (e.g., contrast,sharpness, graininess, acquisition quality and symmetry metrics). Forexample, comparison of overlay, precision, TIS, and TIS variabilityresults obtained by Covariance and Fourier Decomposition methods canserve as one such instrument. Measurement of phase difference betweendifferent Fourier harmonics from the same signal gives importantinformation concerning symmetry of the marks due to processimperfectness, aberrations or illumination problems. Performing the sameanalysis for the target rotated by 180° allows the separation ofasymmetries on the wafer from those due to the metrology tool. Fillingthe whole FOV by target structures allows the selection of differentworking zones, thus providing information about variations within asingle target and allowing additional optimization. Finally, gratingtargets provide an opportunity for simpler diagnostic of the target'stilt in FOV.

Furthermore, although the algorithms have been described as utilizingone dimensional arrays of information, it should be noted that they mayalso be applied to two dimensional arrays of information.

Moreover, although the marks herein have been described for measuringoverlay, they may also be used for one or more of the followingmeasurements or applications: CD, exposure monitoring, resist profilemonitoring, focus monitoring, and the like.

It should also be noted that there are many alternative ways ofimplementing the methods and apparatuses of the present invention. It istherefore intended that the following appended claims be interpreted asincluding all such alterations, permutations, and equivalents as fallwithin the true spirit and scope of the present invention.

1. A method of designing an overlay mark which is used to determine therelative position between two or more successive layers of a substrateor between two or more separately generated patterns on a single layerof a substrate, the method comprising: providing a base overlay markarchitecture including a plurality of working zones selected from atleast first and second working zones that are separately generatedpatterns on a single or successive layers of a substrate, each of theworking zones including a periodic structure positioned therein, each ofthe periodic structures including a plurality of coarsely segmentedelements that are formed from a plurality of finely segmented elements;optimizing the geometry of the working zones of the overlay markaccording to a first scale, the first scale corresponding to a metrologykernel scale based on metrology tool limitations, process issues andcircuit design criteria; optimizing the geometry of the coarselysegmented elements of the overlay mark according to a second scale, thesecond scale corresponding to an image resolution scale based onmetrology tool limitations and process robustness issues; and optimizingthe geometry of the finely segmented elements of the overlay markaccording to a third scale, the third scale corresponding to alithography resolution scale based on circuit design rules, processrobustness issues and metrology tool limitations.
 2. The method asrecited in claim 1 wherein the first, second and third scales aredifferent.
 3. The method as recited in claim 2 wherein the second scaleis smaller than the first scale, and wherein the third scale is smallerthan the second scale.
 4. The method as recited in claim 1 whereingeometry is defined as size.
 5. The method as recited in claim 1 whereingeometry is defined as shape.
 6. The method as recited in claim 1wherein geometry is defined as distribution.
 7. The method as recited inclaim 1 wherein the metrology kernel scale ranges from about 4 micronsto about 10 microns.
 8. The method as recited in claim 1 wherein thegeometry of the working zones is optimized by identifying the upper andlower limits of the first scale and fine tuning the geometry of thefirst geometry between the upper and lower limits.
 9. The method asrecited in claim 8 wherein fine-tuning is implemented by defining theperimeter of the overlay mark and dividing the overlay mark into aplurality of working zones that are configured to diminish the impact ofnon-uniformities across the overlay mark on tool and wafer inducedshifts.
 10. The method as recited in claim 1 wherein the imageresolution scale ranges from about 0.3 microns to about 2 microns. 11.The method as recited in claim 1 wherein the geometry of the periodicstructures are optimized by identifying the upper and lower limits ofthe second scale and fine tuning the geometry of the second elementbetween the upper and lower limits.
 12. The method as recited in claim11 wherein fine-tuning is implemented by balancing the image resolutionof the tool with the process.
 13. The method as recited in claim 1wherein the image resolution scale ranges from about 0.01 microns toabout 0.5 microns.
 14. The method as recited in claim 1 wherein thegeometry of the finely segmented elements are optimized by identifyingthe upper and lower limits of the third scale and fine tuning thegeometry of the third element between the upper and lower limits.
 15. Amethod of designing an overlay mark, the method comprising: defining theperimeter of the overlay mark; dividing the overlay mark into aplurality of working zones, the working zones including at least a firstworking zone associated with a first process and at least a secondworking zone associated with a second process, the second process havingdifferent characteristics than the first process; adjusting the geometryof the working zones, the geometry of the work zones being based atleast in part on a first scale; positioning a periodic structure withineach of the working zones, the periodic structure having a plurality ofcoarsely segmented lines; adjusting the geometry of the periodicstructures, the geometry of the periodic structures being based at leastin part on a second scale, the second scale having characteristics thatare different than the first scale; separating the coarsely segmentedlines into a plurality of finely segmented elements; and adjusting thegeometry of the finely segmented elements, the geometry of the finelysegmented lines being based at least in part on a third scale, the thirdscale having characteristics that are different than the first andsecond scales.
 16. A method of designing an overlay mark, comprising:providing a metrology tool for measuring the overlay mark; providingcircuit design rules for separately generated circuit patterns on asingle or successive layers of a substrate; providing a base overlayarchitecture that is measured by the metrology tool, the base overlayarchitecture including a plurality of working zones that fill theperimeter of the overlay mark, the working zones being selected from atleast first and second working zones that are separately generatedmeasurement patterns associated with each of the separately generatedcircuit patterns, each of the working zones including a periodicstructure positioned therein, each of the periodic structures includinga plurality of coarsely segmented elements that are formed from aplurality of finely segmented elements; selecting a perimeter thatdefines the boundary of the overlay mark, the perimeter being based onat least a characteristic of the metrology tool or a characteristic ofthe circuit design rules; selecting a geometry of the working zonesbased on a characteristic of the metrology tool or a characteristic ofprocesses used to generate the working zones; selecting a geometry ofthe periodic structures within each working zone based on acharacteristic of the metrology tool or a characteristic of processesused to generate the working zones; and selecting a geometry of thefinely segmented elements within the working zones based on acharacteristic of the separately generated circuit patterns.
 17. Themethod as recited in claim 16 wherein the size of the perimeter is setby the field of view of the metrology tool and a scribe line budgetassociated with the circuit design rules.
 18. The method as recited inclaim 16 wherein the shape of the perimeter is selected from squares,circles, triangles, rectangles, or polygons.
 19. The method as recitedin claim 16 wherein the size and shape of the perimeter coincides withthe size and shape of a field of view of the metrology tool.
 20. Themethod as recited in claim 16 wherein the geometry of the working zonesis based on tool induced shifts or wafer induced shifts.
 21. The methodas recited in claim 16 wherein the geometry of the working zones isdetermined by performing optical simulations or experiments of toolinduced shifts.
 22. The method as recited in claim 16 wherein the stepof selecting working zone includes determining the number of firstworking zones and second working zones, the size of the first and secondworking zones, the shape of the first and second working zones and thedistribution of the first and second working zones.
 23. The method asrecited in claim 22 wherein working zones generated together are placedin an opposed relationship.
 24. The method as recited in claim 22wherein the overlay mark includes an equal number of first and secondworking zones.
 25. The method as recited in claim 22 wherein the overlaymark includes a disparate number of first and second working zones. 26.The method as recited in claim 22 wherein the shape of the working zonesis selected from squares, rectangles, circles, triangles, or polygons.27. The method as recited in claim 22 wherein the shape and size of theworking zones are identical.
 28. The method as recited in claim 22wherein the shape and size of the working zone are different.
 29. Themethod as recited in claim 16 wherein the geometry of the periodicstructures are configured to find the proper balance between the imageresolution of the metrology tool and the process robustness of theoverlay mark.
 30. The method as recited in claim 16 wherein the geometryof the periodic structures are determined by experimentation.
 31. Themethod as recited in claim 16 wherein the step of selecting periodicstructures includes determining the number of coarsely segmentedelements within each periodic structure, determining the linewidths ofthe coarsely segmented elements within each periodic structure,determining the spacings between the coarsely segmented elements withineach periodic structure, and determining the pitch of the periodicstructures.
 32. The method as recited in claim 31 wherein the pitch ofthe periodic structures are selected to minimize tool induce shiftvariability and process variability.
 33. The method as recited in claim31 wherein geometry the periodic structures located within the first andsecond working zones are selected with the same characteristics.
 34. Themethod as recited in claim 31 wherein geometry the periodic structureswithin the first and second working zones are selected with differentcharacteristics.
 35. The method as recited in claim 16 wherein thegeometry of the finely segmented elements are selected to mimic the lenspattern placement error of the circuit pattern.
 36. The method asrecited in claim 16 wherein the geometry of the finely segmentedelements is substantially equal to the geometry of the circuit pattern.37. The method as recited in claim 16 wherein the geometry of the finelysegmented elements within each of the periodic structures are the same.38. The method as recited in claim 16 wherein the geometry of the finelysegmented elements within each of the periodic structures are different.39. The method as recited in claim 16 wherein the shape of the finelysegmented elements is selected from lines, squares, rectangles,triangles, polygons, circles, or ovals.
 40. The method as recited inclaim 16 wherein the step of selecting the geometry of the working zonesincludes separating the perimeter of the overlay mark into fourquadrants, and placing at least two separately generated working zoneswithin each of the quadrants, the at least two separately generatedworking zones substantially filling the quadrant.
 41. The method asrecited in claim 16 wherein the step of selecting the geometry of theworking zones includes separating the perimeter of the overlay mark intofour quadrants, and placing at least one working zone within each of thequadrants, the working zones substantially filling the quadrant, theworking zones in diagonally opposed quadrants being generated together.